In Zoo Tycoon 2, one of the expansion packs has programming that prevents animals related to each other from mating. At first, it sounds like Fridge Logic; after all, animals do often mate with their relatives. But think about it: in the game, one of the goals is to breed healthy offspring, as is one of the goals of many real life zoos. And in real life, zoos do not allow animals too closely related to each other to mate, even going so far as to perform genetic tests on them.
Why are some of the animal models reused? Because animal families are related. There isn't all that much biologically different between a Lion and a Leopard.
In the Zoo Tycoon Marine Mania expansion pack, you're able to create mermaid exhibits. The mermaids will flirt for visitors and act relatively human, but place a shark in with them... It makes you wonder if this is why there aren't any mermaids.