They actually joke about it in the show, screaming "Aah, a crocodile", and it's just some woman working there. Same with the director being a lion, I believe.
"Come on kids, over here are the accountants and these are the people who think it's fun to run marathons. Look at them run! Aren't they silly? They're- Billy! Get away from that Republican! You know they bite!"
My theory is that though humans died out, evolution carried on with other animals leading to the shows cast of human-like characters. But then there is still Santa Claus.
Additionally, vampires exist and are canon. Their teacher is one of them.