I don't think a bestiary is strictly necessary for the game, but as a side project for someone who'd be up for doing it that could gather what's been brought in so far (pokemon, chocobos) along with what might have already been there and what's native that's been mentioned already (the flying things that attacked the city during migration) with the caveat that 'this list is not complete or incapable of being added to', it could be a nice addition. But considering that you're doing so much work as is on your own, I personally think that this would be a project best delegated to someone with the desire, drive, and time to manage it. Because it isn't entirely necessary, but it could also be helpful and fun in general.