During the episode where Jake meets Lady Rainicorn's parents. Misinformation happens and he accidently gives her parents permission to eat Finn. They then attack him in a blind hunger-rage and explain that humans are now extinct and are a delicacy to them.
The question is why? Were they considered that before the mass extinction or because of it? And is it only Rainicorns that like to eat humans, or do other species of Ooo desire the taste of human flesh?