Personally, I’m fine with extinct animals AS LONG as humans had the chance to interact and study them correctly before their disappearance. Why?
Animals we know that existed, but can only be reproduced through bones or other leftovers (including vague descriptions and mentions in old writings) are constantly changing for us. We don’t have a tangible proof of how they were or how they acted because we couldn’t never study them properly. As a result, ever few years we discover something new that changes again all we believed an animal was. Our perception of dinosaurs or mammoth as changed several times in the last years, and it’s certainly something that won’t stop happening. After all, the point is that we can only speculate.
I love Zoo Tycoon 2, but if the info on the animals is clearly outdated, the info on extinct species is VERY noticeable wrong. Specially dinosaurs, that I’m not sure were even trying to be accurate on the first place. It will be so out of places in Planet Zoo, that is aiming to have realistic animals and behaviors. Well, you can’t be realistic if you don’t even know how reality was for the extinct species.
Recently extinct species are fine to me. We have good studies, drawings, photos and even records of them in captivity; so they won’t get them wrong. And although it’s not realistic, I’ll argue it’s educational, teaching about how many animals were extinct in so little (relatively) and how they are going extinct now, and it’s a real thread to lose awesome animals if we don’t protect them.
Marine animals would be cool too, but I would leave out most controversial points like whales and shows.
And fantasy would be awesome...in another game. There’s not a clear consensus on any of these animals for obvious reasons, it will only create fights between people because everyone will want their version of the mythical creature (or the version on their favorite fandom) and there’s simply no way to choose who’s right, because everybody and nobody is.