Okay, so I've told you hints of the Prehistoric Beasts, but now, I have made the entire backstory of their origin, along with how old the planet is and using resources from past Sonic material.
Millions of years before Sonic the Hedgehog was born, there lived a time when anthropomorphic dinosaurs roamed and conquered the planet. However, the dinosaurs were led peacefully by Terror the Emperor, along with his royal subjects Armor the Defender and [insert the name of the Spinosaurus character here]. With all good, there is the horrible; in this case, the planet was not always peaceful, as wars constantly erupted for land or to claim the crown. However, one day, there was something much more dangerous than a war; the planet broke apart. In the time of the planet breaking, Terror and his royal guards attempted to rescue his dinosaur citizens, but most were lost by erupting fault lines. Only three dinosaurs civilians were saved; a war veteran (Thrust the Charger), an ace-pilot [insert name of the pterodactyl character here], and a regular citizen [whatever dinosaur I end up choosing for this character]. Realizing that they might die, Terror decides to let the five others stay in the royal castle (now dubbed as the "Temple of the Beasts") and they all wait patiently for the earthquakes to stop. Eventually, a powerful earthquake had caused the castle to crumble, thus presumably killing the six dinosaurs. This event is titled "The Extinction".
Millions of years later, a war between Knuckles, the guardian of Angel Island, and the Angel Island Egg Army, led by Thrash, is currently happening for the fate of the ownership of the island. The Angel Island Egg Army specializes in archeology, and a research team finds bones of the fabled-Axolotl Tribe. Thrash, the Egg Boss of the Angel Island Egg Army who solely joined the Eggman Empire was to defeat Knuckles (who he has a vendetta against Knuckles for the Knuckles Clan stealing the land of the Tasmanian Devil Tribe (this land is eventually known as Angel Island, before it rose)), attempts to use the bones of the Axolotl Tribe to recreate a new super soldier with an equal amount of strength to face and defeat Knuckles. Using the Revival Ring, Thrash successfully revives Slither, and trains him to be a formidable foe against Knuckles. Knuckles finds out the presence of the new Egg Army, travels to Hydrocity Zone to find the Egg Army Base (leaving the Chaotix in charge of the Master Emerald), and is ambushed by Slither. Slither, told that his kind was killed by the Knuckles Clan, fights against Knuckles, but Knuckles convinces him that his kind wouldn't do such a thing as that, to his understanding, at least. While not mostly convinced, Slither decides to call a truce with Knuckles, as the echidna promises that he'll prove that they weren't responsible for their extinction. They invade the Egg Base, and while Knuckles attempts to find the man in charge, Slither goes to the computer room and finds out that his kind was killed by Dark Gaia waking up from his ten thousand year slumber. Meanwhile, Knuckles is getting his butt kicked by Thrash, who only vaguely reveals his vendetta against Knuckles. Realizing his kind wasn't killed by the Knuckles Clan and that he was told a lie by Thrash, Slither helps Knuckles by distracting Thrash, acting like he still serves Thrash. Slither decides to take Knuckles and throw him out, but Slither reveals his true colors, subdues Thrash, and escapes with a barely-conscious Knuckles.