Snail

The Snail is a character that the creators of Adventure Time hide in every episode (excluding "Trouble in Lumpy Space" and "Blood Under the Skin") . He's seen making cameos, usually waving at the screen. His cameos are a running gag. Snail also transports himself in Finn's bag as we can see in "Mortal Folly".

The Lich
While the Snail was just a running gag at first, he eventually released The Lich, the most powerful evil in the Land of Ooo. In the episode "Mortal Folly," he comes out of Finn's bag to wave to viewers, then falls under The Lich's mind-controlling spell and releases him from his amber prison. 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, in a deep, rumbly voice, although a deep voice may be a side effect of being possessed by the Lich, as Princess Bubblegum had a deep voice while possessed too. The Snail used the Bear to get 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 One
The tables below list the Snail's known whereabouts.

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

Season Four
In season 4, The possessed snail is still making cameo appearances but, played a major part in episode 7.

Trivia

 * Its running gag of appearing at a certain moment, usually hidden, could be a reference to "Where's Waldo?" It's also similar to Pandaman of One Piece series, and the Black Cat in the Trigun series. It is also similar to the Bloody GIRs from Invader ZIM, the "Hidden Mustachios" in the "Annoying Orange" series, Derpy Hooves, Lyra Heartstrings, Bon Bon, and Carrot Top in My Little Pony: Friendship is Magic, Goldbug" from the children's books by Richard Scary, and it even shares its form with the hidden snail of Blue's Clues.
 * Multiple times in the third season, the Snail appears normal and not possessed by The Lich. This simply may be because the animators thought of it to be so small, nobody would notice (Mostly able to be seen like this in "Too Young," "Fionna and Cake," and "What Was Missing").
 * 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 really huge because they were mutants.
 * In "His Hero," and "Wizard Battle" only his shell is shown.

Episode appearance statistics

 * "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.
 * "Holly Jolly Secrets Part Two," "Ghost Princess," "Dad's Dungeon," and "Princess Monster Wife" all have had the snail in the preview.
 * The snail hasn't appeared in only two episodes ("Trouble in Lumpy Space" and "Blood Under the Skin"). The snail was later added in "Blood Under the Skin." This makes the snail the third most shown character, behind Finn and Jake.
 * The Snail appeared in the title card of "Mortal Recoil".

Appearances in other media

 * In Righteous Quest 2, the Snail appears a couple times in the game.
 * The Snail starred in its own FusionFall game: "Snail Quest."
 * The Adventure Time 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.