Leviathan

Leviathan was a whale that appeared in the episode "Billy's Bucket List". In the episode, Finn knocks himself out with a wooden board so he can overcome his fear of the ocean. Because of knocking himself unconscious, he has a stange dream where Leviathan appears and steals his hat by biting it's "ear" and pulling it off. At the end of the dream, Leviathan eats him, leading to Finn being woken up by the Fear Feaster.

Appearance
Leviathan appears as a normal whale, but with, apparently, no eyes. It's skin is a dark shade of blue on the top, and a lighter shade on its underside. However, towards the end of Finn's dream, Leviathan appears purple. Leviathan also has sharp teeth, unlike some whales, allowing it to eat Finn easily.

Trivia

 * Leviathan's name was most likely inspired by the biblical creature of the same name