Like said in the previous replies, I must agree. The godmodding, trolling, violence that would come with it would be horrific.
Plus, I think this game was supposed to be somewhat realistic, yes.. we do have some neon creatures running around, but ---- Oh nvm, I'll just get to the point.
If we had dinosaurs in FH, they'd most likely have to be huge to be realistic, and that would be somewhat annoying. I don't think you would want to see nothing else than a giant foot on your screen every time you go closer to a dino, am I correct? xD
This game was based on carnivores, so I don't see how birds will ever be here. But of course.. If you want to have a bird character so much, just make a tiny canine or feline with wings.
Humans.. no no no.. what if some of them would just go running around with weapons and killing the actual animal RPers, I imagine how that would turn into a disaster.
That's why you have the great games like Skyrim, Aro'Kai(although it's a WIP game) and many others.