Snail

The Snail is a character that hides in all episodes, usually seen waving at the screen. His cameos are a running gag. In the episode "Mortal Folly," The Lich possesses him to break itself free. However, in "Mortal Reciol," Princess Bubblegum (possessed), passes the Lich into him.

The Lich
While the Snail was just a running gag at first, he eventually became an integral part of the story when he released the lich in the episode "Mortal Folly." The Snail comes out of Finn's bag to wave to the viewers, but then falls under the Lich's mind-control spell and breaks the amber prison by bashing it with his head. He appears at the end of "Mortal Recoil," still being controlled by the Lich.

At the end of "In Your Footsteps," the Snail speaks for the first time and gains The Enchiridion. The Snail still continues to appear in each episode, and (when examined closely) used to have green eyes and an evil expression on its face. The possessed snail was voiced by Pendleton Ward.

In the episode, Jake the Dog, The Lich than possessed Billy. In the next episode, Up a Tree, Finn and his squirrel friend, flew over the snail and he no longer had green eyes and the evil expression on his face. He only did a friendly smile and waved to Finn and his squirrel friend.