I'm not too fussed either way on the idea of having a bestiary or not, I've personally just made stuff up when I needed to, but I don't think it's necessarily a bad idea either to give people ideas for what to encounter on missions or in threads. I do think it should be something that you as a mod don't have to maintain by yourself, just because that's a lot of extra work for something that not everyone will even use or care about.

Personally I think the best way to do it would be to have a post with a few guidelines (if needed) at the top and maybe a note that creatures listed in the bestiary are not the ONLY things out there, you can still make up your own monsters or bring in canon ones, etc, and then have players add to it in comments. That way you just have to get it started and maybe fill in the first handful of entries (pokemon, chocobos, whatever). But you wouldn't have to be solely responsible for maintaining it going forward, people could add creatures to it as they come up in their threads.