In the real world, Prehistory is very long. The formation of the earth, that whole thing with the dinosaurs, then eventually the evolution of humans all took place in prehistory. Neanderthals, who were another humanoid species similar to humans, existed entirely in what we will define as “prehistory” for our purposes: The period before anything existed which was smart enough to remember what happened, and to tell someone else about it. Prehistory can blur into Mythos quite a bit, but that’s perfectly fine. When defining you setting you only need a general sequence of events; separating portions of history is simply a helpful way to work through the timeline.
Your world’s prehistory might be extremely long, like that of real-world Earth, or it might be relatively short. Settings with extant deities might have very short prehistories which end as soon as the divine beings appear to start remembering things. Settings without superhuman forces might have long earth-like histories.
Your world’s position in the timeline of prehistory doesn’t need to be at any one point. Your setting’s prehistory might predate your world’s creation by a great deal. An alien race may have populated the galaxy, and may have created the world for some purpose. Millenia of historical events may have played out before your world came into existence, some of which might be discovered by archaeologists, but much of which may never be know.
If you world is the first in your setting to support intelligent life, your pre-planetary prehistory might solely entail how the universe came to be, or it might be left as an intentionally unanswered question.
Example 1 – Shadow of Olympus
Greek Mythology’s prehistory is very simple. There was some vague chaos, and somehow the first titans (Gaea, etc.) formed from the chaos and proceeded to create the rest of the natural forces, titans, and gods which populate greek mythology.
This plays very well into the information we have previous established for Shadow of Olympus. The “chaos” is elemental chaos which encircles the material plane, so we can say that prehistory is the undefined period before the first titans sprang into existence and began to solidify the planes.
Source: Greek Mythology.com – The Creation.
Example 2 – Space Grease
Since Space Grease is a real-world future setting, we can steal real-world Earth’s prehistory.
Example 3 – Heroes of Tonesvale
Heroes of Tonesvale is a alternate-Earth setting, so we can steal most enough of Earth’s prehistory that defining additional information isn’t very interesting.