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 the Lich free. However, in "Mortal Recoil," 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) appears to still have green eyes and an evil expression on its face. The possessed snail was voiced by Pendleton Ward.

Season 1
The tables below list the Snail's known whereabouts.

Season 3
In season three, the snail is still possessed by the Lich in every cameo appearance.

Season 4
In season 4, the possessed snail is still making cameo appearances, but it played a major part in the episode, "In Your Footsteps."

Trivia

 * The Snail's eyes are below where an actual snail's eyes are.
 * The Snail is the only snail of his size that has appeared in the show. Other snails have been shown, but they were much larger.
 * "Storytelling," "Mortal Folly," "Mortal Recoil", and "In Your Footsteps" were the only episodes in which the snail was featured as part of the story.
 * Finn, the Lich, the Bear, and Shelby are the only characters who have noticed or interacted with the snail.
 * Multiple times in the third season, the Snail appears normal and not possessed by the Lich.
 * "Holly Jolly Secrets Part Two," "Ghost Princess," "Dad's Dungeon," "Princess Monster Wife," and "Who Would Win" all have had the snail in the preview.
 * The Snail appeared in the title card of "Mortal Recoil."
 * The snail was not originally in two episodes ("Trouble in Lumpy Space" and "Blood Under the Skin"), but it was added in to both episodes before reruns aired.
 * In "His Hero," and "Wizard Battle" only his shell is shown.
 * Similar recurring, hidden characters in other media include:
 * The snail in Blue's Clues,
 * Pandaman in One Piece,
 * The Black Cat in Trigun,
 * The Hidden Mustachios in The Annoying Orange,
 * Derpy Hooves in My Little Pony: Friendship is Magic,
 * The Hidden Mickeys in Disneyland,
 * Tomee Bear in Eddsworld,
 * Visitors in South Park,
 * Bloody GIR in Invader Zim.

Appearance in other media

 * In the game Righteous Quest 2, the snail appears a couple times in the game.
 * The Snail starred in its own FusionFall game: "Snail Quest."
 * The game Legends of Ooo uses snails as hints. Also, a slimy snail is used to free Slime Princess.
 * Issue 1 of the Adventure Time comic series begins with the Lich-controlled snail entering the Bag of Holding, which somehow allows the Lich to return to his old body.
 * In Issue 3 of the Adventure Time comic, the Snail makes two cameo appearances on top of a Snow Finn's head.
 * In Issue 4, the Snail appears in the background when Princess Bubblegum is thanking Finn, and is also seen on Marceline's lifeguard chair in the panel after the Great Mushroom War comic is shown.