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.
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.
“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!”