Snail

The Snail is a character that has hidden in every episode of Adventure Time to date (excluding the Animated Short), usually seen waving at the screen. His cameos are a running gag. In the episode "Mortal Folly," The Lich possesses the Snail to break the amber prison where he is trapped. Then, in "Mortal Recoil," Princess Bubblegum (while being possessed), passes the Lich onto 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," and once again at the beginning of "The Lich," still being controlled by the Lich. In the adventure time comic book series he goes into the Lich's bag still controlled by the Lich and comes out normal again unharmed.

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.

In "The Lich," the Lich ends up changing bodies moving to that of the deceased Billy. After this, the snail no longer shows any further sign of possession as revealed in "Up a Tree."

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." As inferred from the dream in "The Lich," the Snail appears to have been abandoned by the Lich when he took over Billy.

Season 5
With the Lich abandoning the Snail at the end of the previous season, it has been restored to its normal pre-"Mortal Folly" role.

Trivia

 * In the "Animated Short," the snail does not appear.
 * The snail seems to be male, although technically snails are neither male or female. Of course, it's a cartoon, so anything can be possible.
 * The Snail's eyes are below where an actual snail's eyes are.
 * "Storytelling," "Mortal Folly," "Mortal Recoil," "In Your Footsteps," "The Lich" and "Up a Tree" were the only episodes in which the Snail was featured as part of the story.
 * Finn, the Lich, the Bear, Princess Bubblegum, Squirrel, and Shelby are the only characters who have noticed or interacted with the snail in the TV series.
 * In the Adventure Time comic, "Marceline and the Scream Queens," Marceline picks up the snail by a lake, and throws it thinking its a rock, which causes it to skip.
 * 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.
 * On the side of the Jake vs. Me-Mow DVD box containing the Finn Hat, the Possessed Snail can be seen.
 * The Snail was shown longest in "Finn the Human," with a total of 19 seconds, it was shown earliest in "Dad's Dungeon," and was shown the latest in "Holly Jolly Secrets Part II."
 * Also, in "Finn the Human," the Snail is shown as a regular snail.
 * Similar recurring, hidden characters show up in other media, an example of which is the snail in Blue's Clues.
 * The idea of the snail appearing every episode is similar with the black cat appearing every episode in Trigun.

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.
 * In Adventure Time with Fionna and Cake Issue 1, the Snail can be seen in the cover of the comic waving.