in may 2011 lots of pets that were stuck in auctions and other places were glitched, resulting in basically a loss of their birthday, so they became prehistoric ^^
Creatu Become Prehistoric in two ways. One, they were born in V1. I wasn't around at that time, but I guess they didn't have Birthdays back then, so when they were introduced, all those V1 Creatu were labeled as Prehistoric. The second way is that they are moved via the admin panel from one part of the site to another. This became common when Creatu would get stuck in the Auctions after receiving no bids and a Staff member had to move the Creatu from the auctions and back to the owner. This issue is solved now, but because the glitch existed for so long, Prehistorics lost value in quite a few users' eyes.