I think a bestiary could be useful for keeping track of information on creatures that have been established by the players already - making it clear that we can add to it whenever we want, and maybe making it comment-based so that it's on the players to keep it updated, with some general guidelines in place and the ability for you to veto a species that could break the game. That would mean that we'd be keeping track of things like pokemon and AllMates, which ICly are played as a recent addition to the universe, and we'll also have a handy reference for made-up creatures that don't come from any specific canon.

Also, if the bestiary is handled like that, then whichever player posted each creature's data block will be an easily accessible source of information if someone wants to play with that creature but isn't sure if something seems accurate for them behaviorally. At least, that's the thought.