Pathfinder - Practical Guide to Animal Companions
Last Updated: November 26th, 2017
I will use content from the core rules, but will intentionally omit any content not published on the official Pathfinder SRD due to the unmanageable volume of non-SRD content, and the wildly varying quality of non-SRD content. If you would like me to write handbooks for specific content not published on the official SRD, please email me and I will consider it on a case-by-case basis. I will use the color coding scheme which has become common among Pathfinder build handbooks. Also note that many colored items are also links to the Paizo SRD.
- Red: Bad, useless options, or options which are extremely situational.
- Orange: OK options, or useful options that only apply in rare circumstances
- Green: Good options.
- Blue: Fantastic options, often essential to the function of your character.
Temporary Note: Pathfinder Unchained and Occult Adventures were both recently added to the SRD. I'm excited to explore them, and I am actively working on adding their contents to my collection of handbooks. I appreciate your patience while I make these changes.
Animal companions are a key component of druids and rangers, and through archetypes can be components of other classes. Selecting your animal companion is an important choice, and can have major effects on how your character functions on the table. With the right choices, your animal companion can be a scout, a striker, a defender, or a mount, depending on your needs.
Ultimate Campaign includes some wonderful content about how to handle pets in your game. Reading it isn't essential, but it may improve your play experience when pets are involved.
How Animal Companions Work
First, familiarize yourself with the table below, taken from the Druid class entry. As you gain levels, your animal companions abilities grow, and it adds additional hit dice, tricks, feats, and other useful stuff. Keep in mind that Rangers and most other classes will get an animal companion at a druid level equal to their class level -3.
|Class Level||HD||BAB||Fort||Ref||Will||Skills||Feats||Natural Armor Bonus||Str/Dex Bonus||Bonus Tricks||Special|
|1st||2||+1||+3||+3||+0||2||1||+0||+0||1||Link, share spells|
|4th||4||+3||+4||+4||+1||4||2||+2||+1||2||Ability score increase|
|9th||8||+6||+6||+6||+2||8||4||+6||+3||4||Ability score increase, Multiattack|
|14th||12||+9||+8||+8||+4||12||6||+8||+4||5||Ability score increase|
|20th||16||+12||+10||+10||+5||16||8||+12||+6||7||Ability score increase|
Link (Ex): Handling an animal to make them do something normally requires a move action, so if you like to train attack dogs or something, keep that in mind. Allowing you to handle your animal companion as a free action allows you to order them around while still taking your full turn. The +4 bonus to wild empathy and handle animal with your companion makes it considerably easier to teach additional tricks.
Keep in mind that there is actually a DC of 10 to make your companion do something, and a strict DM might make you roll for that. With a few ranks in Handle Animal and that nice +4 bonus, you can be sure to pass the skill check even if your companion is injured (+2 to DC). Between your wisdom, skill ranks, and the +4 bonus, be sure to have a +11 bonus to gurantee that you can always handle your companion, even on a natural 1.
Share Spells (Ex): While not particularly useful for a druid, this allows hilarious things like animal domain clerics casting Divine Might on their companion.
Evasion (Ex): This will keep your animal companion alive. Know it, love it, embrace it.
Ability Score Increase (Ex): This allows you to add 1 to any of your companions ability scores. Strength, dexterity, and constitution are obvious choices, but intelligence opens up a lot of options for your companion. Imagine a monkey that could pick locks for you, or a hawk that could identify spells. Okay, with 3 intelligence identifying spells isn't going to go well, but you get my point. 3 intelligence also allows your companion to understand language, which means you can give it (slightly) complex orders that go beyond whatever tricks it knows.
Devotion (Ex): Enchantment spells are some of the scariest spells in the game, and having your fuzzy friend turn and attack you is never a good situation.
Multiattack: If your companion has multiple natural attacks, multiattack is a fantastic feat. If they don't the free secondary attack is crucial in making up the damage gap between animals with multiple attacks and those with only single attacks. Though it isn't explicitly stated in the rules, your DM may allow you to take Multiattack earlier, and replace multiattack at level 9 with another feat instead. This would be consistent with other bonus feats gained by character classes, but be sure to confirm with your DM before you make any assumptions.
Improved Evasion (Ex): Leaps and bounds better than evasion.
Animal Companion Skills
Animal companions have a limited list of skills unless they have 3 intelligence, and with only 1 skill rank per hit die you don't have a lot of skills to throw around. Pick skills which fit your companions role, and will help them help you.
All of the skills marked with an (*) are class skills for animal companions, so they get the +3 class skill bonus just like you do.
- Acrobatics* (Dex): The ability to jump and tumble past foes in combat can really help your companion. Many animals have respectable dexterity, which can make this work very well.
- Climb* (Str): Climb isn't very useful for players, and putting it on an animal doesn't make it any better.
- Escape Artist (Dex): If your animal companion ever needs Escape Artist, you should be doing something to rescue it.
- Fly* (Dex): Situational. If your animal companion has a fly speed, it gets a +8 to fly. Maybe consider one more rank for the super rare Fly check, but don't bother with more than that.
- Intimidate (Cha): Animals typically have average charisma, but many of them are also large. Their lack of available skill ranks makes this difficult, but the idea of a big gorilla standing behind you glaring at people makes for some interesting games.
- Perception* (Wis): The most rolled skill in the game, and you companion has an extra set of eyes to put behind a roll. Regardless of your other selections, it is hard to justify bypassing the +3 bonus for spending a single rank on this.
- Stealth* (Dex): Most animal companions won't be sneaky, but this is great for ambush predators like snakes.
- Survival (Wis): If your animal companion has scent, and many do, they get a bonus to track by scent. This can allow your animal companion to be a tracker for the party. This can also allow for amusing things like sending your horse to track your enemies.
- Swim* (Str): Situational.
Animal Companion Feats
Your companion gets several feats as you progress in level. Like skills, your selection of feats depends heavily on your companions role. because it is an animal, it has a limited selection of available feats until it reaches 3 intelligence.
- Acrobatic: Inless you companion spends a ton of time tumbling, not worth a feat.
- Agile Maneuvers: If you companion has high dexterity and Grab or Trip, this is a must.
- Armor Proficiency (light): One of your companion's biggest problems will be its AC, and light armor won't slow it down.
- Armor Proficiency (medium): If your companion isn't a mount and it doesn't need a lot of speed, medium armor is a good choice.
- Armor Proficiency (heavy): If your companion is a dedicated defender, heavy armor can be a good option, but can be expensive.
- Athletic: Bonuses to two very situational skills.
- Blind-Fight: Because many animal companions have scent, they are excellent at locating invisible enemies. This helps them to then kill or grapple those enemies.
- Combat Reflexes: If your animal companion is a dedicated defended, this can be a good option. Keep in mind that many animal companions who work well as defenders won't have enough dexterity to make use of Combat Reflexes.
- Diehard: This can keep your animal companion in a fight, but personally I would rather see my animal companion take a nap until it gets healed.
- Dodge: Additional AC is crucial for your animal companion.
- Endurance: Highly situational.
- Great Fortitude: Animals already have good fortitude saves, and many have good constitution scores.
- Improved Bull Rush: If you companion is large and has sufficient strength, this can be a good option.
- Improved Initiative: Animal companions act on your initiative, so this has no effect.
- Improved Natural Armor: A good option for a defender, but not as good as dodge or armor proficiency.
- Improved Natural Attack: The damage payoff on this feat is rarely worthwhile, so don't rush to get this.
- Improved Overrun: If your animal companion is being used as a mount, this is a good option. Overrun allows you to move over and past unimportant targets to reach high priority targets the rear. Overrun also allows you to potentially damage some targets and knock them prone.
- Intimidating Prowess: Intimidate is available to animals, though it isn't a class kill. With this, a decent strength, and a large animal companion you could have a pretty good intimidate check.
- Iron Will: Your animal companion won't draw as much attention as actual party members, but Will is your companion's worst save.
- Lightning Reflexes: Better reflex saves makes evasion more effective, and keeps your companion alive through fireballs and such.
- Mobility: Mobility is great for pushing past front-line targets to reach more important targets at the rear, or for escaping.
- Power Attack: Power Attack is an excellent source of damage, but keep in mind that your companion gets Druid BAB, and doesn't progress in hit dice as quickly as you. Power Attack may give up too much of your companion's attack bonus, and won't scale very quickly.
- Run: Very situational.
- Skill Focus: Very situational.
- Spring Attack: Very situational, but it can be a lot of fun to use.
- Stealthy: Very situational.
- Toughness: Additional hit points will keep your companion alive, but remember that they don't progress in hit dice at the same rate that you do, so they won't get an extra hit points every time you gain a level.
- Weapon Finesse: Animals with Dexterity higher than their Strength get Weapon Finesee for free as a bonus feat.
- Weapon Focus: For striker companions, Weapon Focus can help to add much-needed attack bonus. Because they get druid BAB and don't get a hit die every time you gain a level, companions may have trouble keeping their attacks accurate as you level.
In addition to the feats taken by your Animal Companion, you can take feats which will enhance your Animal Companion.
- Animal Soul: There are very few spells which only affect animal companions.
- Evolved Companion: Many 1-point evolutions are fantastic, and you can take this feat multiple times. Get Reach for Defender companions or Pounce for striker companions that don't already have it. There are lots of fantastic options.
- Improved Spell Sharing (Teamwork): This is good for defensive buffs that you and your animal companion don't need, like Resist Energy.
- Pack Flanking: You don't need flanking as much as someone with Sneak Attack, but the +2 to hit is nice.
- Share Healing: Situational.
- Spirit's Gift: Many of the Spirit Animal buffs are very good for an Animal Companion. Bones grants constant concealment, Fire grants immunity to fire damage, Heavens grants very limited flight, Life grants fast healing, and Stone grants DR 5/adamantine. Unfortunately it only works for a few minutes per day.
Can I ride my companion?
To the best of my knowledge, there isn't a hard rule on this. Obvious choices like Horses, Ponies, Camels, and Riding Dogs are clearly meant to serve as mounts, so we can try to draw some parallels to make good rulings on what might be allowed to be ridden. A mount should meet the following criteria:
- At least one size larger than the rider
- Capable of carrying the rider's weight (Enough Strength to lift the rider)
- Horizontal riding surface of some kind on which to seat the rider
- Rigid body (so no oozes)
These are not solid rules, obviously, but they are good guidelines. An ostrich, for example, could make a reasonable mount. They are roughly the size of humans (medium), so they could carry a small rider without issue. An ape, even if it were large, would make a poor mount because it does not have a horizontal riding surface to saddle, and the way they move is not conducive to riding. As always, check with your DM and come to a reasonable solution.
The closest that I have found to an official list of rideable animals is the Beast Rider Cavalier's list of mount choices. If you are uncertain, use the animals on that list as examples.
Does my Animal Companion gain Reach when it becomes Large?
Unfortunately, that's specific to each creature, and Paizo has not provided any official guidance on reach for animal companions. Michael Brock, the global organized play coordination over at Paizo, responded in this forum thread simply to say that the Large Ape animal companion has 10 foot reach, but provided no other insight regarding reach for animal companions.
Based on the other suggestions in the same forum thread which Mr. Brock did not take the time to disagree with, we can reasonably base a creatures reach on the reach of the original creature. If an Animal Companion's base creature was Large and had reach, it can be assumed that the Large Animal Companion would therefore have reach.
Ruling on Animal Companions whose base creature was bigger than Large is somewhat more difficult. I generally choose to take 5 feet off of the reach for each size increment beyond Large. Therefore, a Huge creature with 15 foot reach would reasonably have 10 foot reach at large size. Similarly, a Gargantuan creature with 20 foot reach would have 10 foot reach at large size. There are some notable cases where creatures have considerably more reach than their size would typically indicate, such as the Diplodocus, which gets a stunning 60 foot reach in its typical Colossal form (30 foot square). Your GM may see the hazard in offering 20 foot reach to large creature, but even 15 foot reach is quite generous.
As always, check with your GM and come to a reasonable conclusion before making any rules assumptions or character choices.
Selecting Your Animal Companion
When selecting your animal companion, keep in mind what role you want them to fill. If your party is light on armor, you may need a defender. If your party has problems with damage output, you might need a striker. If your build requires mobility, you might want a mount. Some choices have interesting combat options like Grab and Trip which can provide additional utility in battle.
Also note that most animals have low-light vision and scent, so we can consider those two senses a base line for animal companions. Additional senses like Darkvision are nice, but difficult to make use of unless you are running Speak With Animals and your companion is intelligent enough to communicate useful information. Additional movement modes are also nice, especially if your companion is large enough to carry you.
|Allosaurus||M||Forest (warm), Plains (warm)||B2|
|Ankylosaurus||M||Forest (warm), Plains (warm)||B1|
|Antelope||S||Plains (temperate, warm)||B3|
|Archelon||M||Coastlines (warm, temperate), Water (warm, temperate)||B3|
|Aurochs||M||Buffalo or bison||Plains (cold, temperate)||B1|
|Axe beak||M||Plains (temperate)||B3|
|Bat, dire||M||Any Terrain (temperate, warm)||B1|
|Bird1||S||Any large birds such as eagles, hawks or owls||Mountains (temperate)||CRB|
|Brachiosaurus||M||Forest (warm), Plains (warm)||B1|
|Cat, big||M||Lions, tigers||Plains (warm), Forest (cold)||CRB|
|Cat, Small1||S||Cheetahs, leopards||Plains (warm), Forest (warm)||CRB|
|Centipede, giant||S||Forests or Underground (temperate or warm)||UM|
|Chameleon, giant||M||Forest (warm), Mountains (warm)||B3|
|Crab, Giant||S||Any aquatic||UM|
|Crocodile||S||Alligators||Rivers/Lakes (Swamp (warm))||CRB|
|Dimorphodon||S||Coastline or Forest (warm)||B4|
|Dolphin||M||Ocean (cold, temperate, warm)||B1|
|Eel, electric||S||Warm fresh water||B1|
|Elephant||M||Mastodons or mammoths||Plains (warm)||B1|
|Elk||M||Plains (temperate, cold)||B3|
|Frog, giant||M||Swamp (temperate, warm)||B1|
|Gar||M||Marshes and Aquatic (temperate, warm)||B2|
|Gecko, giant||S||Forest (warm), Mountains (warm)||B3|
|Goblin dog||S||Forest (temperate), Swamp (temperate), Underground||B1|
|Iguanodon||M||Forest (warm), Swamp (warm)||B3|
|Kangaroo||M||Hills (temperate), Plains (temperate)||B3|
|Leech, giant||S||Marsh (temperate or warm)||UM|
|Manta ray||M||Ocean (warm)||B2|
|Mantis, giant||M||Forest (temperate)||UM|
|Megalania||M||Forest (warm), Plains (warm)||B3|
|Megaloceros||M||Giant elk||Forest (temperate, warm), Plains (temperate)||B2|
|Megatherium||M||Forest (temperate, warm)||B2|
|Monitor lizard||S||Forest (warm), Plains (warm)||B1|
|Moray eel, giant||M||Ocean (warm)||B1|
|Octopus||S||Ocean (cold, temperate)||B1|
|Orca||M||Killer whales||Ocean (cold)||B1|
|Pachycephalosaurus||M||Plains (warm, temperate)||B3|
|Parasaurolophus||M||Forest (warm), Plains (warm)||B2|
|Scorpion, giant||M||Deserts, forests, plains, or underground (warm or temperate)||UM|
|Shark1||S||Ocean (cold, temperate, warm)||CRB|
|Slug, giant||M||Marshes or Underground (temperate or warm)||UM|
|Snake, constrictor1||M||Any kind of constrictor snake||Rivers/Lakes, Swamp (warm), Forest (warm)||CRB|
|Snake, viper1||S||Any kind of poisonous snake||Any Terrain (temperate, warm)||CRB|
|Snapping turtle||M||Coastline, Rivers/Lakes, Swamp (temperate, warm)||B2|
|Spider, giant||S||Any (warm)||UM|
|Spinosaurus||M||Forest (warm), Swamp (warm)||B3|
|Squid||M||Ocean (cold, temperate, warm)||B1|
|Toad, giant||M||Forest (temperate), Plains (temperate, warm), Swamp (warm)||B2|
|Tylosaurus||M||Ocean (warm), Rivers/Lakes||B2|
|Tyrannosaurus||M||Forest (warm), Plains (warm)||B1|
|Vulture, giant||M||Plains (warm), Hills (warm)||B3|
|Wasp, giant||M||Forests (temperate)||UM|
|Wolf1,2||M||Any, Forest (temperate)||CRB|
A passable striker right out the gate, the Allosaurus gets three attacks (though the claw attacks are secondary), and has an AC of 17. With 40 speed it can get around the field easily to reach key targets.
At level 7 the Allosaurus gets becomes large and gets a all of the usual size adjustments to abilities, adds Grab and Pounce, and probably adds 10 foot Reach. Pouncing with 22 strength (actually 24 due to the animal companion bonus to strength/dexterity) and 3 attacks is going to do a whole lot of damage.
With a starting natural armor bonus of +9, the Ankylosaurus starts off at 21 AC, and can make a fine defender if you're careful about its hit points. The 9 constitution is a bit of problem unless you improve it, but with some armor and Toughness, the Ankylosaurus can stand alongside your fighter and potentially have a higher AC with little cash investment.
The addition of large size and the usual size adjustments, reach, and Stun at level 7 makes the Ankylosaurus a great area control defender, and the bonus to strength/dexterity from the animal companion table makes Combat Reflexes very viable. The ankylosaurus has 2 intelligence, which makes it easy to make it intelligent, but be sure to invest in its low constitution, and boost it strength to raise the DC on Stun. Avoid the Stand Still feat because it is considerably less effective than Stun. At this size, you might be able to use the Ankylosaurus as a mount. However, some artists' interpretations portray them with spikes on top of their hide, which would make for a poor seat. Consult your DM.
The giant ant brings a climb speed to the table, which can make it a good choice for a mount for small creatures at level 4 when it becomes medium. It has decent constitution, but its natural armor is low and its strength and dexterity will make it unlikely to hit. It has Grab, which would be great if it wasn't small with 10 strength.
Level 4 makes the giant ant somewhat more useful, but doesn't fix its low natural armor, and only slightly improves its strength. It will still have trouble hitting, and at medium with 14 strength Grab won't get it very far. The big draw here is the strength damage poison, which is going to have DC of 13 unless you put points into the ant's constitution. The strength damage is fairly small, but there doesn't appear to be a limit on how often it can be used, and you can always harvest poison from it to give to your rogue.
Too small to be a mount, too little damage to a striker, not enough stealth to be a scout. This thing is a fast-moving bullseye.
Apes have a climb speed and become large, but I don't think I would try to use one as a mount. They get three attacks, but fairly low natural armor and not much constitution. They have 2 intelligence, which means they can easily be made intelligent, and have three attacks. They make a decent striker, but won't do much damage until level 4 and they need help with their defenses.
Level 4 brings large size, reach, and switches their strength and dexterity. At 21 strength, thos natural attacks start to look pretty scary, and the extra constitution goes a long way to improve the ape's defenses. Even with these improvements, be sure to invest in armor and other defenses to keep you ape alive. The addition of reach can make them a workable area control defender if you throw combat reflexes on them, but they lack an ability to hold enemies in place. Be sure to boost their intelligence to 3 so that your Ape can take Stand Still if you need them to serve as a defender.
An Archelon is a pre-historic turtle. Look them up, they're super cool. An Archelon starts at medium size, which should hint at the fact they were enormous, and therefore cool. Despite their slow land speed, these guys can really get around in water, and have a phenomenal +10 natural armor. Sadly, that's really all that they bring to the table. Their strength and dexterity are awful, and their constitution isn't good enough to make them a tank.
This is a prehistoric relative of the elephant and the rhinoceros, and its stats show it. It has respectable natural armor, strength, and decent constitution to back it up. I would also consider this a viable mount.
Level 7 brings powerful charge and trample, which really make this a good mount option for mounted charge characters. Trample adds a free attack to overrun, and the damage from powerful charge is added to the normal damage for a charge attack. Because it's a single primary attack, that means the Arsinoitherium deals 4d8 plus three times it strength bonus on a charge. With a minimum of 22 strength, that's 4d8+18 damage on a charge. Even if you can't use it as a mount, that is a very impressive striker.
Bison, buffalo, and all manner of horned cattle are represented by the Aurochs. Their speed is respectable and their ability scores are acceptable, but their natural armor is weak, and they damage is poor.
Level 7 doesn't improve the Aurochs much. The boost to strength is great, but with a single lackluster gore attack, it's not going to get you much. Trample is also nice, but it's still not enough to make this a good option. Stampede will likely never come up unless you find a way to draw your enemies into herds of cattle, at which point you probably don't need your animal companion.
A fictional flightless bird with a huge, axe-shaped beak. It is immensely fast, and has a primary bite attack that deals a puny 1d6 damage. With its starting strength, that is all it gets. It doesn't even get a natural armor bonus.
4th level brings is a splash of natural armor, and finally a decent strength score in addition to large size. Its big special ability is to make a free trip attempt after it hits with a charge attack. I have seen people ride ostrich, and those are only medium, so it stands to reason that a large Axe Beak might make for a passable mount. The added benefit of tripping on a charge might even be enough to draw a mounted charge character.
Small, no natural armor, no special move type, mediocre stats, only one attack with poor damage, and its 4th level advancement doesn't meaningfully improve it.
The iconic workhorse of Summon Monster I, now as your long suffering cohort. Badgers bring a climb speed and a very rare burrow speed to the table. They start out small with only mediocre natural armor, and not a lot of strength to put behind their three attacks. They get to rage for 6 rounds a day, but they're only enough to get you through one or two very short fights.
Level 4 brings medium size, and increase strength and damage. Their natural armor doesn't improve, and the combined size increase and loss of dexterity means that they lose two points of AC. The added damage is nice, but they still only get 6 rounds of rage in a day, and that's not enough to make this animal companion helpful. Unless you particularly need a burrow speed, look elsewhere.
Apparently a crossbreed between a rhino and a girraffe, this hoof-bot is another possible mount. It has good natural armor, and decent stats. Its hoof attacks are somewhat lacklustre, but it has 40 foot speed.
Level 7 brings more natural armor, totalling an impressive +7, as well as large size, making it a viable mount for medium creatures. The improved strength is great, and Trample makes Overrun a better option. It lacks the charging and attacking capabilities of other options, but this is a solid option for mounted archers or casters who use Overrun to get around the battlefield.
A big, scary, prehistoric predatory whale. Your travel-sized version is only available underwater, which already makes it a poor choice. Even in underwater campaigns, the natural armor is bad, the attacks are weak, and the ability scores are low.
Level 7 improves the natural armor just enough to offset the AC lost from the size increase and dexterity loss, but the improved strength brings some much-needed damage. The biggest draw here is the 60 foot blindsense. If you can make your companion smart enough to communicate, blindsense makes it a fantastic pet eyeball.
A 40 foot fly speed with good maneuverability is pretty great. At medium size, it might even be a viable mount for a small character. The attack is weak, especially with only 9 strength, but with 9 constitution and no natural armor, this thing needs to stay out of combat. It can serve as a scout or a mount, but damage is not in the dire bat's wheelhouse. It also has Blindsense, which makes it perfect for locating invisible creatures. If you can make it smart enough to communicate, this can make it a good spotter for spellcasters or archers who like to fight from the air.
Level 7 brings some much-needed natural armor, some strength which you won't ever use, some extra consitution, and of course large size. Large size makes it a potentially viable mount for medium creatures.
Despite small size, the bear has impressive speed, a little bit of natural armor, three attacks, and respectable ability scores. With some armor, it could make a passable defender or striker. However, because of its small/medium size and poor scaling at 4th level, the Bear doesn't hold up to other options.
4th level brings medium size and minor improvements on what the bear is already doing fairly well. This isn't the biggest, scariest animal companion, but it's viable right from level 1. At medium size a bear be a mount for a small character, but keep in mind that bears tend to stand up when they get into fights.
No low-light vision or scent, but you get darkvision. The Giant Beatle is slow, and has a slow fly speed with poor maneuverability, but it has a solid +6 natural armor. The bite damage isn't going to get very far with such poor stats, so this thing is an annoying ball of natural armor that sometimes flies into your windows repeatedly for hours on end. Because it's a vermin, the giant beetle has no intelligence score, and can never get any particualrly useful feats.
Level 4 brings medium size, slightly improved stats, and a really lousy trample attack. Keep in mind that trample only works on small creatures, so your beatle is only going to be trampling halflings.
The very specifically named "Bird" is a catch-all for birds of prey (I think). They get an impressive 80 foot fly speed, and three attacks with fairly poor damage. With no Scent, these are mostly only useful as flying eyes. You can use them to fly in and flank with you, but with only 12 con and negligible natural armor, your Bird is going to get eaten by something bigger and scarier. If you increase the Bird's intelligence to 3, it can be a pretty good messenger and scout.
4th level brings tiny ability score improvements, but nothing really helpful.
Real boar are only small size, and if you see one in the wild you very quickly run in the other direction and hope it isn't following. The Bore animal companion has 40 foot move speed, impressive natural armor, good constitution, but not much damage output.
Level 4 brings a tiny bit more damage, medium size, and Ferocity. Ferocity is basically Direhard for animals, which means your boar will continue doing its best to get killed after someone has put it in negative hitpoints. Oh, and you could probably ride it now if you're small.
Medium size out the gate, and a possible mount option for small characters. 30 foot speed isn't awful, and the Brachiosaurus gets decent natural armor, and a respectable tail attack.
7th level makes the Brachiosaurus large, and a possible mount for medium characters. The added natural armor offsets the size increase, and additional strength makes that tail attack more viable. Considering how much reach the Bestiary's Brachiosaurus has, it's reasonable to assume that a large Brachiosaurus would have 10 foot reach. However, with its weak defensive stats it doesn't make for a great area control defender. Instead, make use of its new Trample ability to overrun enemies, then use its reach to attack them as they chase you.
A horse designed by committee. It has less natural armor than a horse, but it has better ability scores. It also has less attacks than a horse, but it gets a spit attack which you might find helpful. Sickened is almost as good as Shaken, and the reduced saving throws can be very helpful for a clever spellcaster. Spit is a ranged touch attack with no saving throw on the sickened effect, so be sure to make use of it at every opportunity.
Level 4 brings some tiny ability score improvements. Keep spitting.
Lions, tigers, panthers, etc. The Big Cat is a striker. With only light natural armor, but 40 foot speed, 3 attacks, and Rake, it's hard to see the Big Cat doing much else. At medium size they might make be a mount for small riders.
Level 7 brings large size, and even more striking capabilities. Additional strength, Grab, and Pounce to throw it all behind. Large might also make it a potential mount for medium creatures, but cats don't seem like they would make for a pleasant ride. Still, combining their pounce ability with a mounted charge character would make for a truly fearsome pair.
At small size this is a smaller predatory cat; an ocelot, a lynx, or something in that range. The small cat trades the Big Cat's size and damage for Trip, greatly improved dexterity, and increased speed. This is still a striker, but less of a scary damage striker, and more of a support striker.
Level 4 brings medium size, a tiny bit more damage, and Sprint. Sprint is great on a trip striker, as it allows them to run a huge distance and tackle your escaping foes while the rest of your party rushes to catch up.
Giant Centipedes are slow, but have a climb speed. Their natural armor is light, and they don't have great constitution, so they can't really take a hit. The cetipede's big draw is its poison. The DC is con-based, so at small size the DC is only 10, and the dexterity damage from the poison is pretty small. As a vermin, it gets Darkvision instead of Low-light vision and Scent, and it has no intelligence score.
Level 4 brings medium size and some small improvements to ability scores.
The chameleon has average speed and a matching climb speed. It has no natural armor, and not much in the way of combat stats. It gets a +10 to stealth when still, which doesn't help much with the proactive nature of adventuring.
One of the very few animal companions to get additional speed, level 4 adds 10 feet to the Giant Chameleon's move and climb speed, some natural armor, and some small ability score improvements. The chameleon also gains Tongue, which allows it to Grab a target and pull them toward it. This is a nice ambush mechanic for medium or smaller creatures, but with the chameleon's low strength and poor bite damage, it's not going to do much on its own.
The Giant Crab has average speed and a slightly slower swim speed. It gets two claw attacks with very little damage, but it also gets Grab and Constrict. At small size it can't really hurt much. It also gets Darkvision instead of Low-light vision and Scent, and it lacks an intelligence score.
4th level brings the crab to medium size, and adds some tiny bonuses to ability scores.
Even at small size the crocodile is a respectable defender and striker. It gets decent natural armor, and has very good ability scores, plus a bite attack with Grab. However, it has a slow move speed and doesn't get scent. With some armor, the crocodile can lock down single targets very well. However, it only has 1 base intelligence, which makes it hard to make it intelligent.
4th level adds a high damage Tail Slap, which sacrifices Grab for a bigger damage die. The added ability scores and medium size help with Grab, but the real winner here is Death Roll, which makes it a much scarier grappler. The crocodile also gets Sprint, which helps make up for its slow speed when you need it to charge.
Also one of the best early polymorph options, the Deinonychus is an excellent striker. It has a stunning 60 foot speed, but very little natural armor, and not much strength at small size. Still, it has a wopping 18 constitution and respectable dexterity, so at the very least it will survive to see 7th level.
7th level is very good for the Deinonychus. It grows to medium size, gets a bit of extra ability scores, and adds two talon attacks to its existing 3 natural attacks. On top of its now 5 attacks, it also gets pounce. With a bit of focus on its strength, this can lead to a very impressive amount of damage.
Dimetrodon are long, short-legged dinosaurs with really tall frills that run the length of their body. The Dimetrodon gets a small amount of natural armor and decent ability scores. Its gimick is that its bite attack has a 19-20 critical threat range. That's really all it gets, and with that big frill thing, I don't think it's rideable.
7th level brings large size, and not enough natural armor to offset the size change. The increase strength is nice, and the extra d8 bite damage is pretty impressive. Still, the Dimetrodon lacks any real interesting abilities, and doesn't do enough damage to make it a good choice.
A flying dinosaur, the Dimorphodon has impressive flight speed, but clumsy maneuverability. Compared to the Bird, it gets slightly more natural armor and scent, but less strength and far worse maneuverability.
At level 4, the Dimorphodon gets slightly better ability scores, and poison. The poison deals strength damage and has a con-based DC which starts at 12 and deals strength damage. Even so, this thing doesn't have the stats to be in melee poisoning people.
Another quadrupedal, long-necked, herbivorous dinosaur. Based on its stats, we can compare it to the Ankylosaurus. The Ankylosaurus has worse constitution and slightly less tail slap damage, but considerably better natural armor. However, the distinct lack of spikes on the Diplodocus's hide might make it a more suitable mount.
Level 7 brings large size, more natural armor (though not enough to catch up to the Ankylosaurus), considerably more tail attack damage, and better ability scores. Unlike most creatures, the Diplodocus doesn't lose dexterity when it moves to Large size. The Tail Lash ability also allows the Diplodocus to make two attacks with its already impressive tail. Based on the Bestiary 5 Diplodocus entry, a large Diplodocus might have 10 or even 15 foot reach, which would make it a viable area control defender even without the Ankylosaurus's Stun ability. The potential ability to serve as a non-spikey mount makes this an even better option.
Smaller, weaker, and slower than the Wolf, and it doesn't get Trip. If you really need your canine to get through civilized areas without suspicion, bluff people and tell them that your dog is a wolf-hound, and hope they don't have Knowledge (Nature).
Very fast in the water, dolphins aren't very impressive in combat. Their big appeal is blindsight 120 feet once they hit 4th level, which turns them into very vast, very powerful underwater scouts.
The only appeal of the Electric Eel is the additional electricity damage which it deals when when it advances at 4th level. Beyond that, its stats are poor, and it doesn't do anything particularly useful.
A bite attack with move speed and a swim speed. The best case you can make for the Elasmosaurus is that it might get reach when it advances because of its exceptionally long neck. Even with its 2d6 bite damage after advancing, the Elasmosaurus's damage still isn't very impressive.
While not listed as a mount, Elephants are used as mounts throughout Southern Asia, and have been used as war mounts for thousands of years. With 40 speed, decent natural armor, and respectable stats, Elephants are decent right from the start.
7th level brings the Elephant to large size, improves the gore damage to an impressive 2d6, and adds 8 more to its already respectable strength. Combined with its new Trample ability, the Elephant is an excellent overrun mount, and it van get by on its good natural armor and constitution to serve as a defender or even a striker. With this much versatility, and viability from level 1, the Elephant is an excellent choice for nearly any character and nearly any situation.
Elk get the same speed as a horse, and seem like a possible mount option. Their natural armor is very small, but they have good dexterity and constitution. Their attacks are fairly weak, and they have to choose between their gore or their hoof attacks. They also don't get scent for some reason.
Level 7 brings large size and some numerical improvements, but doesn't actually make the elk any more interesting. The attacks do a bit more damage, but the Elk still lacks any interesting abilities like Trample which other similar options get.
The Giant Frog has average speed and a matching swim speed. Its natural armor is very weak, but it has good ability scores to start with. It also starts with Tongue and Pull, which allow it to grab targets 15 feet away and pull them into a grapple.
Level 4 provides very little in the way of improvement. Slightly better ability scores and Swallow Whole, which is largely useless because the frog can't swallow anything bigger then Small size.
At medium size, this is an impressively large fish. It has paper thin natural armor, but it has good ability scores and a bite with grab. It lacks a land speed, but in the water it's fast and can be a passable grappling striker.
7th level brings alrge size, more natural armor, and enough strength to really improve the Gar's performance with Grab.
I'm confused why the Giant Gecko doesn't just have a climb speed. Expert Climber allows it to climb (at half its land speed) on any vertical surface, which is effectively a slow climb speed. On top of that, it lacks scent, has poor ability scores, and no other interesting abilities. 4th level brings more speed and medium size, but meaningful improvement.
A new-born giraffe is roughly 6 feet tall, making it medium size. This means that either you found an orphaned giraffe or walked right up to its mother and were like "yo, I'm taking this". The giraffe has speed equal to a horse, two respectable hoof attacks, and impressive strength even at medium size. It has a little of natural armor and great dexterity, but it doesn't have a lot of constitution to back up its low AC.
Level 7 brings an additional slam attack (Giraffes hit things with their long necks. Look it up.), large size, and some improvements to ability scores. The Giraffe's strength doesn't improve as much as most creatures moving to large size, but a total strength of 20 is still pretty good. Because of its clearly horse-like shape, you might consider using a giraffe as a hilariously long-necked mount, but I would advise against this as giraffes are somewhat spindly and they are clearly not build to carry weight on their backs.
The Goblin Dog has excellent speed, but fairly terrible stats. It gets the iconic Allergic Reaction, but even that only applies a very minor ability penalty. 4th level brings medium size, and a net loss to AC. The ability score improvements are nice, but still not enough to make this a orange choice.
Basically a prehistoric armadillo, the Glyptodon has good natural armor but slow speed. It gets two claw attacks with respectable damage, but its ability scores are fairly bad. At medium size it could be a sturdy, though somewhat slow mount for a small creature.
Level 7 brings large size and all of the typical associated benefits. The increase strength makes the Glyptodon considerably more dangerous, but its natural armor only increases enough to offset the size change. The glyptodon can now serve as a very slow mount for a medium creature, or a somewhat mediocre defender.
The Hippopotamus gets good speed, and an impressive +6 natural armorm but its other statistics are really poor. Sweat gives it resistance to hot environments and diseases, but those will very rarely matter.
7th level brings the hippo to large size, doubles its bit damage to an impressive 2d8, adds all of the ability score improvements that come with large size, and adds Trample. The addition of Trample makes the hippo a rather amusing potential mount. Combined with the hippo's respectable strength and impressive bite damage, it could make a passable overrun mount. Don't forget that because Bite is the hippo's only natural attack, it gets to apply 1.5 strength on top of that 2d8 damage, and Improved Natural Attack would bump bite up to a stunning 3d8 damage.
The Horse is one of very few animal companions which starts at Large size and is immediately ridable by medium creatures. It has excellent speed, and impressive +4 natural armor, and good ability scores off the bat. 4th level brings some small ability score improvements and Combat Trained, which makes the horse an excellent combat mount. The Horse doesn't get Trample, which is disappointing, but with Combat Trained it can still be an excellent mount choice for any character that needs one.
A smaller, more dextrous version of the wolf. The Hyena's big gimmick is Trip, which can be done faster by a Small Cat, and more effectively by a Wolf. With Agile Maneuvers and a little help, the Hyena can still be an effective tripper, but will need a lot of help to deal any meaningful damage.
Iguanadons start at medium size, and would make for a fine mount for a small creature despite only having average speed. Their ability scores are very impressive with a starting strength of 17. The Iguanodon only gets on claw attack, which is dissappointing, but it gets to put 1.5 strength behind it. It also gets a little bit of natural armor, and has enough constitution to keep it standing.
Level 7 brings large size, all of the normal size changes to ability scores, and Thumb Spikes, which makes the Iguanadon's claw equivalent to two-handing a battleaxe. At a very impressive 25 strength (not counting the animal companion bonuses), that claw deals 1d8+10 damage, which is very respectable. Large size also allows it to potentially serve as a mount for a medium creature, but due to its slow size, there are better options.
The Kangaroo has good move speed, but otherwise poor stats. Its only natural attack does very little damage, especially with so little strength. 4th level brings medium size, slightly better stats, a tiny bit of natural armor, and 10 more feet of move speed. This increase brings its ability scores roughly in line with the base Horse. With no special abilities and no chance of being a viable mount, the Kangaroo really has nothing to offer.
With 5 foot move speed, no natural armor, and poor stats, the Giant Leech is a very squishy bag of hit points. Blindsense is fanatastic, but because the leach is a vermin it lacks the capacity to communicate. Level 7 adds a rather horrifying Blood Dragin special attack, and improves Blindsense to Blindsight, but with so little move speed the leech will have trouble catching aything to latch onto.
Even underwater, this is strictly worse than the Dolphin. The only advantage to the Manta Ray is that they reach Large size and might serve as a mount.
With average move speed, a climb speed, and a fly speed, the Giant Mantis has a lot of good ways to get around. Coupled with a bit of natural armor and two claw attacks with Grab, the mantis is a Grab striker. As a vermin, it lacks and intelligence score and trades Low-light Vision and Scent for Darkvision.
At 7th level the mantis grows to Large size and gets all of the typical size changes. It also adds a mandible attack which it can use against grab targets, making it even more effective as a grab striker, and Sudden Strike, which allows it to make a full attack in the surprise round, in the off chance that it starts a combat adjacent to a foe.
Something like a pre-historic crocodile, the Megalania has average move speed and paper thin natural armor. Its dexterity is good, but that's its only really great ability score. Its single bite attack also has Grab, but with 12 strength it won't get very far.
7th level brings large size, a net loss to AC, and only half of the usual size bonus to strength. It also gains a poison for which no stats are provided. If it keeps in line with other animal companions with poison, it should deal 1 dexterity damage per round for 6 rounds, with 1 save to cure, and a con-based DC equal to 10 plus the Megalania's constitution modifier.
This might as well be called Dire Elk. It has 50 foot move speed, decent natural armor, and decent ability scores, but only a single gore attack. Compared to the standard elk, its gore attack does more damage, but it gives up the two hoof attacks. The Megaloceros also has two more points of natural armor.
7th level brings large size, bringing the Gore attack to 2d6 damage, and adds two secondary hoof attacks, far outclassing the Elk's need to choose between its gore attack or its hoof attacks. Assuming that it can be used as a mount, this makes the Megaloceros roughly equivalent to a Horse with slightly adjusted ability scores.
A giant, pre-historic sloth, the Megatherium has good move speed (especially for a sloth), and a very slow climb speed. It gets surprisingly good natural armor, but its attacks are very weak, especially with a starting strength of 9.
7th level brings Large size and all of the normal ability adjustments, but also adds Rend. If the Megatherium's ability scores weren't so poor, it might make a decent striker.
Small and Grab are not a good combination. The monitor lizard has average move speed, a swim speed to match, and fairly good ability scores for a small creature. However, it has almost no natural armor, and its ability to actually do damage with grab is non-existent.
7th level brings medium size, slightly better abilities, and a poison on top of Grab. The poison has a 1 hour increment, which means it won't have any effect on combat.
Moray Eel, Giant
Starting off at medium size with a respectable +5 natural armor, good bite damage and Grab, the moray eel is an aquatic Grab striker. Even at medium size it has great physical stats.
7th level brings large size, more natural armor, 2d6 bite damage, and all of the size bonuses to ability scores, bringing the eel to an impressive 24 strength. Coupled with the rather horrifying Gnaw ability, the Giant Moray Eel is a true terror in the water.
Even in the water, the octopus isn't great. It has average swim speed, and an impressive Jet speed, but its damage is pathetic, and its tentacles give it grab so that it can wave its dismal strength score around. 4th level adds poison to the bite attack, but even this doesn't really make the octopus useful.
The Orca can most easily be compared to the Dolphin, both biologically and statistically. Surprisingly, the Orca is more dexterous than the Dolphin at start, and has slightly less strength and constitution. Their natural armor is equal (and bad), and the Orca's bite is slightly more damaging than the Dolphin's slam.
7th level brings the Orca up to Large size, and dramatically increases its strength and constitution. Continuing to compare to the dolphin, the Orca now has the same AC, 5 more strength, 2 more dexterity, 1 less constitution, and a much bigger natural attack. The Orca also picks up blindsight 120 feet at this level, the same as a dolphin. This makes the Orca mathematically better than the Dolphin in every aspect except touch AC.
A bipedal dinosaur with a built-in football helmet, the Pachycephalosaurus is a bull rush with legs. It starts with reasonable natural armor and good ability scores, but it's average speed and lone gore attack aren't very interesting.
Level 7 brings large size, and all of the normal size benefits, including 10 foot reach. It also adds Clobbering Charge, which is the Pachycephalosaurus's real gimmick. Clobbering Charge adds a free Bull Rush at the end of a successful charge attack. Coupled with the Pachycephalosaurus's impressive 23 strength and large size, it has a very high chance of success on this bull rush attempt. If you want Bull Rush to be part of your overall combat strategy, this is a great option.
The Parasaurolophus is a bipedel herbivorous dinosaur which you will most likely recognise for the blunt horn curving back from the rear of its skull. The Parasaurolophus gets a bit of natural armor, average speed, and impressive dextertiy, but its stats are otherwise poor.
7th level brings large size, probably 10 foot reach (the Huge Parasaurolophus has 15 foot reach), and all of the normal size changes to ability scores. It also adds Tample, making the Parasaurolophus look like a possible Overrun user. Despite being bipedal, it also looks like it would make a solid mount. However, it's fairly poor stats, average speed, and poor natural armor make this a fairly poor choice for any purpose.
The small character's Horse equivalent. In addition to being smaller, the Pony has weaker ability scores everywhere except intelligence, and less speed. You think the smaller size might at least give it slightly better dexterity. It even has two less points of natural armor! 4th level brings the same benefits that the Horse gets, which means the pony remains statistically inferior. If you need a mount for a small character, consider a Wolf, a Dog, or a Small Cat.
Clumsy fly speed, no natural armor, and a whole mess of dexterity. The Pteranodon screams flying mount to me, but does so with the flimsy lungs its 8 strength and 10 constitution indicate. Despite being slightly faster and having a bigger damage die, the Pteranodon is strictly worse the the Dire Bat. 7th level brings Large size, but even this leaves it lagging behind the Dire Bat's superior stats and blindsense.
The Ram is a bull rush with legs. It has 40 foot speed, almost no natural armor, and at small siz with 10 strength won't be doing much of anything until it advances at 4th level. 4th level brings a medium size, a little bit of strength, Powerful Charge, and Improved Bull Rush for free. This still isn't much, especially considering the existence of the Pachycephalosaurus.
Dire Rats get decent move speed, as well as climb and swim speeds. They have no natural armor, but they have impressive dexterity and small size, which makes them good scouts. If you increase their intelligence and can speak with animals, their scouting ability becomes truly impressive.
4th level brings some really pointless ability improvements, and adds Filth Fever to their bites, which will likely never matter in a fight because diseases take days to do anything.
The rhinoceros has a respectable +4 natural armor, and really good ability scores at start. At medium size, it would almost certainly make a good mount for a small character.
7th level brings Large size, which bumps the Rhino's gore attack up to 2d6 damage, adds all of the usual size bonuses to abilities, and an extra point of natural armor on top of it. The rhino also gets powerful charge for an additional 2d8 damage (probably plus 1.5 strength), which makes its charges very impressive. In addition to being a sturdy mount, it can make for a passable striker.
The description for this animal companion outright states that it works as a mount at large size for medium characters. With +5 natural and 80 foot fly speed, and decent dexterity, it has a great starting AC, but watch out for the 9 constitution. The animal companion entry also fails to list a maneuverability, but it seems logical to assume Average maneuverability based on the Bestiary's Roc entry. The roc also gets three primary natural atacks, but with so little constitution and strength, it won't do much damage. Also keep in mind that the Roc does not get Scent.
7th level brings large size, with all the normal size adjustments to ability scores, plus an extra point of natural armor. This thing is a flying tank.
Finally a scary vermin companion, the Giant Scorphion has good speed at 40 feet, but paper thin natural armor, and fairly weak ability scores. Its appeal comes from the two claws with grab and the string with a poison right from level 1. The poison has a constitution-based DC, and deals strength damage. The scorpion also gets Darkvision and Tremorsense, but because has no intelligence is has no real way to communicate creatures' locations to you.
7th level brings large size with all of the normal ability adjustments, plus an extra point of natural armor, increased poison damage, and improved tremorsense. With now 20 strength and large size, the grab on those claws is pretty impressive. This makes the giant scorpion a powerful Grab Striker, and if your DM is a little crazy it might even serve as a mount.
Yes, an actual seahorse, not a horse that lives in the sea. The Seahorse has a respectable +4 natural armor, and good ability scores off the bat. It improves at 4th level much the same way Horses and Ponies do, and adds the Anchor ability. It is a logical, if somewhat dull, choice for an underwater mount.
One of the few underwater choices to get scent, the shark also gets has a swim speed of 60, +4 natural armor, and fairly good ability scores for a small creature. With poor starting strength, its bite attack isn't very impressive.
4th level brings medium size with the normal ability score adjustments, and adds 30 foot blindsense. Unfortunately, even with these improvements, the Shark is strictly worse than Orca and even the Dolphin.
The Giant Slug is a very odd creature. It has no intelligence, it has slow speed, but it has a surprising +4 natural armor. Its physical ability scores are fairly poor, but it also gets blindsense 30 feet, and some DR. Its real draw is its Spit Acid special attack. Despite being a ranged touch attack, which is the easiest attack in the world to make, it will miss often due to its 8 dexterity.
7th level brings Large size, but very little in the way of ability improvements. The Spit Acid damage increases to 1d8, but the loss of dexterity and increased size makes it even less likely to hit. Its blindsense increases to blindsight, but with no way to communicate the location of targets, this doesn't really help anyone.
When I think Grab Striker, I think Constrictor Snake. Its speed is only 20 feet, but it has matching climb and swim speeds. At medium size, and with good ability scores, it can set Grab to good work right away. However, with only 1 intelligence, it's difficult to make it intelligent, which makes it much harder to pick up feats to improve its grappling capabilities.
Level 4 brings Large size a bit earlier than usual, with all of the normal ability adjustments, but 1 less point of natural armor than normal for a net loss of AC. The snake also picks up constrict, which makes its Grab ability that much more potent. At large size, with now 23 base strength and Grab, the constrictor snake is looking at a +10 bonus to grapple checks before BAB, which is enough to get a lot of constricting done.
The viper is a size smaller than the Constrictor Snake, but has the same speed and bite damage. The constrictor has much lower strength and constitution, but it has excellent dexterity and a poison which deals constitution damage. Unfortunately, because the poison DC is constitution-based, it will be very poor until the snake advances at 4th level.
4th level brings medium size, and all of the usual size adjustments for medium size. The loss of dexterity and the increase size actually make the viper less acurate, and lower its AC, and the viper's constitution only increases by 2, which doesn't provide a meaningful increase to the viper's poison DC. Despite all of the potential of a constitution damage poison, the viper gets actively worse when it advances, making it a very poor choice.
The Snapping Turtle brings an impressive +10 natural armor, but at medium size has poor speed, dismal ability scores, and an insignificant bite attack. 7th level brings Large size with all of the usual ability score adjustments, and adds grab to the turtle's bite. The addition of Grab makes the turtle look like a Grab Striker, but with so little move speed and still very low strength, the Turtle isn't going to be very successful.
The Giant Spider has average move speed and a matching climb speed. It has no natural armor, dismal strength, and only 10 constitution. Its saving graces are a high dexterity, darkvision, tremsorsense, and a strength damaging poison. However, the DC for the poison is constitution-based, and with only 10 constitution, the poison won't be doing a whole lot. In addition, because the giant spider is a vermin, it has no intelligence score and can't be made intelligent.
By level 4 you have picked up Weapon Finesse so that the spider has a remote chance of hitting something, so medium size actually makes it less effective. Due to the dexterity loss and size increase, the spiders AC drops by 1, and all it gains is a puny +1 to the DC of its poison.
The Spinosaurus is about as straightforward as Strikers get. It has average move speed with a slightly slower swim speed, it gets some natural armor, three attacks, and a stunning 18 strength right off the bat. Couple with its other impressive ability scores, this thing is an impressive fighting machine.
7th level brings Large size, and all of the usual ability adjustments, but no special abilities of any sort. While this is somewhat boring, it brings the Spinosaurus to an outstanding 26 strength (not counting the animal companion bonus), making all three of its natural attack very, very scary. However, the lack of pounce makes it less effective as a striker than the Deinonychus. With some armor, the Spinosaurus could easily serve as Defender, but using it as a mount seems unlikely due to the spiny frills for which is is named.
At medium size this is a very large squid. It has a swim speed of 60 feet, a jet speed of 240, and a couple of natural attacks. Its ability scores aren't great, and the squid's big draw is the Ink Cloud ability. While this ability is certainly amusing, it's not terribly helpful. 4th level brings some really meaningless ability score increases.
An even worse version of the Elk. Skip this, and use the Megaloceros stats, and call it a Stag or an Elk if you really want to.
The Stegosaurus has a respectable +6 natural armor bonus, and a big pile of dexterity at medium size. It has average speed, but its constitution and strength are lacking. Despite all this, its tail attack does an impressive 2d6 damage at this size.
7th level brings Large size with all of the usual ability adjustments, and adds Trip to the already impressive Tail attack. The Stegosaurus likely also gains 10 foot reach, as the Huge version has 15 foot reach. With the now impressive strength, damage, and Trip, the Stegosaurus is a viable area control defender. Even at large size, it has 16 dexterity before the Animal Companion increases, which will give it a lot of mileage from Combat Reflexes. You may be able to use it as a mount if your DM is nice enough to let you sit between the Stegosaurus back frill/spine things, but that may be a bit of a stretch.
The Manta Rays smaller, scarier cousin, the Stingray is Small, and slower than the manta Ray, but has a rather impressive poison which deals both dexterity and constitution damage. However, the Stingray begins with low ability scores, so the DC on the poison won't be spectacular, and with only 1 intelligence it is difficult to bring the Stringray's intelligence up to 3.
Level 4 brings medium size, Blindsense, and some modest size adjustments to ability scores. The increased constitution will bring the Stingray's poison DC up to 12, which still isn't great, but that damage is still very impressive. The addition of Blindsense can also allow the Stingray to scout for invisible creatures.
The Styracosaurus has a respectable +6 natural armor, a decent Gore attack, and average speed, but otherwise poor ability scores. 7th level brings large size, with all of the usual size adjustments to ability scores. The Styracosaurus also adds Ferocity so that it will die in combat more frequently, and Reflexive Strike which allows it to make 3 additional attacks of opportunity. Because it now has only 11 dexterity (before the Animal Companion bonuses), Reflexive Strike is important for the Styracosaurus to serve as an area control tank. However, its damage is still poor compared to several other options, and its lack of reach and a crowd control ability make it ineffective at its only interesting gimmick.
The Thylacine is a small, cat-like marsupial. It has startlingly good constitution for such a small creatures, but is otherwise largely unremarkable. Its Powerful Jaws ability improves the threat range of its bite attack, but with only 12 strength behind a d4 bite, it's not really going to do any damage. The poor thing doesn't even get Scent.
4th level brings the Thylacine to medium size, and adds some natural armor. Now with 20 constitution, this thing is a tiny annoying bag of poorly armored hitpoints. With no damage output, no AC, and no useful scouting abilities, this thing has no discernable purpose.
The Giant Toad closely resembles the Giant Frog, but comes with a different, and more useful set of special abilities. It has average speed and a matching swim speed, and decent ability scores. Unfortunately, it has only one point of natural armor.
Level 4 brings some minor ability score improvements, Poison Skin, and Swallow Whole. Poison Skin will help protect your Toad against monsters and people who enjoy licking giant amphibians, but won't be useful offensively. Swallow Whole is a fantastic way to outright murder something, but it only affects creature smaller than your Medium sized Giant Toad. If you plan to murder a lot of small animals or halflings, the Giant Toad can serve as a reasonable stiker.
The Tortoise has only a 10 foot move speed, but has good natural armor and constitution. However, with its dismal dexterity its AC is still terrible. It has a bite attack with fairly poor damage, but good luck finding anyone slow enough for it to matter.
7th level brings Large size, a pile of additional natural armor, and about half of the usual ability score improvements for large size. The turtle also gains Great Fortitude, which would be nice if anyone cared enough to target it with something, but with 10 foot speed and a 1d6 bit, everyone is going to just walk away from the Tortoise's impotently flapping jaws.
Iconic and spiky, the Triceratops seems like a fine mount choice, and makes an obvious charge striker. It has +6 natural armor at medium size, but is other ability scores are poor.
7th level brings large size, an extra point of natural armor, and Powerful Charge. Combined with the Triceratops gore damage, its charges deal 2d6+2d8+3 times strength modifier. Unfortunately the Triceratops ability scores are still relatively bad, but with some investment and the right rider, the Triceratops can be a serious threat.
Someone looked at the Bird option, and decided that they wanted something less predatory and more artistic, but not with appreciably different stats. The Trumpeter Swan is almost identical to the Bird in every way, but is slower and has very slightly less Charisma. If you want a bird, get a Bird.
A big angry prehistoric fish, the Tylosaurus is one of few aquatic options with both Scent and a land speed. At medium size it has a bit of natural armor and impressive dexterity, but its Bite has essentially nothing behind it.
7th level brings Large size, and all of the usual ability score adjustments. The Tylosaurus also picks up Grab, which turns it into a Grab striker. However, with such poor ability scores it will have trouble competing with other Grab strikers like the Cosntrictor Snake.
Iconic, big, and toothy, the Tyrannosaurus has average move speed, and decent natural armor. At medium size it has decent strength and dexterity, but very little constitution. Despite being known for having a particualrly puny brain, it also has two intelligence.
7th level brings Large size with the usual ability score adjustments, an extra point of natural armor, Grab, Powerful Bite, and probably reach. Powerful Bite adds some extra power to the Tyrannosaurus's new 22 strength and 2d6 bite damage, and Grab turns it into a big, scary Grab Striker.
The Tyrannosaurus can most easily be compared to the Allosaurus and the Deinonychus. The Tyrannosaurus gets one big, scary attack with Grab. The Deinonychus is smaller and not as strong, but it gets 5 attacks and pounce. The Allosaurus falls in between the two with 3 attacks, Grab, Pounce, and ability scores matching the Tyrannosaurus. All three are fantastic strikers, but display different focuses and different ability sets. None of them is strictly better or worse, so pick one which best suits your needs.
STAT BLOCK MISSING FROM SRD.
Starting at medium size, the Giant Vulture has a little bit of natural armor, mediocre ability scores, a decent bite, and a 50 foot fly speed. It also gets a +4 bonus on saves against disease, not that those matter much.
7th level brings large size, and all of the usual ability score adjustments. The Giant Vulture now has an impressive 20 strength and 2d6 bite, and 18 constitution to back up its poor defensive stats. It might make a decent flying striker, but it won't win any long-term fights against tough opponents. You might even be able to use it as a flying mount.
The Giant Wasp has an impressive 60 foot fly speed with Good manoeuvrability, and a little bit of natural armor. Offensively, its big option is its poison, which does dexterity damage. As a vermin, it has no intelligence, and has Darkvision instead of Low-light vision and Scent.
7th level brings Largse Size, which puts some much-needed damage behind the Wasp's sting, and improves the poison DC by 2. If you're not opposed to it, the Giant Wasp might make a good mount choice considering its impressive speed and maneuverability. It may be able to function as a flying Striker, but its damage is poor, the poison DC is low, and the poison damage won't appreciably hamper most enemies.
The giant weasel has average speed and a 10 foot climb speed. It gets a single point of natural armor, and has a huge amount of dexterity, but otherwise unremarkable ability scores. The Giant Weasel's big scary gimcik is Blood Drain. Even at only 1 point, the Giant Weasel is draining constitution damage, which piles up very quickly over several rounds. Coupled with Grab, the Giant Weasel is a high risk Grab Striker due to its Small size and poor strength.
4th level brings medium size, and a bit of much-needed strength. This helps quite a bit with the weasel's CMB for Grab/Grapple, but it may still have trouble staying attached to use Blood Drain. With some intelligence and the right feats, the Giant Weasel can be used to weaken very scary foes with big pools of hit points. The Giant Weasel stat entry also mentions that small creatures train them as mounts, which seems like perfect evidence that they could do the same for you.
The Ranger's most iconic choice, the Wolf is a solid option. It has good speed, a bit of natural armor, good ability scores, and Trip right from level 1. It is strictly superior to the Dog in every way except public relations.
Level 7 brings large size with all of the usual ability score improvements, which helps to keep the Wolf a viable Trip monster. Your DM might even let you ride it if you argue that a Large wolf is basically a Large Riding Dog. Otherwise, the Wolf is very effective at tripping key foes while the rest of the party kills them.