I still like the idea of prehistoric-looking effects. What if it was just an effect and had nothing to do with the creatu's birthday being "prehistoric"? Some kind of event could be created in order to get it, and it could be applied to any creatu, even ones born after the glitch.

It could be applied to creatu that are aged 1000 days  or older, independently of the fact that the creatu's birthday is prehistoric or not :).