Snail

The Snail is a character that is hidden in every episode of Adventure Time to date (excluding the animated short and "Food Chain"), usually seen waving at the screen. His cameos are a running gag. In the episode "Mortal Folly," The Lich possesses the Snail and uses it to escape containment in the amber prison. Later in "Mortal Recoil," the Lich possesses the Snail again after releasing Princess Bubblegum from his control.

The Lich
While the Snail was just a running gag at first, he eventually became an integral part of the story when he releases 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 while still under his control 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 episode "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.

Season 4
In season 4, the possessed snail is still making cameos, 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

 * The snail's eyes are below where an actual snail's eyes are.
 * In "The Jiggler," the snail seems to have a different appearance.
 * It has been told that The snail leaves subliminal messages into the minds of viewers, and comes into their dreams, a similar effect of staring The Lich into his eyes.
 * In the episodes "Storytelling," "Mortal Folly," "Mortal Recoil," "In Your Footsteps," "The Lich," "Up a Tree," and "Sad Face," 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.
 * However in "Fionna and Cake" and "The Prince Who Wanted Everything" the snail appeared in Ice King's story and Lumpy Space Princess' story, meaning both may be aware of its existence as well.
 * In the Adventure Time comic, Marceline and the Scream Queens, Marceline picks up the snail by a lake, and throws it thinking it is 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 II," "Ghost Princess," "Dad's Dungeon," "Princess Monster Wife," "James Baxter the Horse," "Red Starved," "Who Would Win," "Apple Wedding," "Lemonhope Part 1," "Billy's Bucket List" and "Breezy" 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 Finn's hat, the possessed snail can be seen.
 * The snail was shown latest in "Holly Jolly Secrets Part II," it was shown earliest in "Dad's Dungeon," and was shown the longest in "Finn the Human (episode)," with a total of 19 seconds.
 * Also, in "Finn the Human," the snail is shown as a regular snail (without eyes, a mouth, and arms).
 * Similar recurring, hidden characters show up in other media, an example of which is the snail in Blue's Clues.
 * In "Simon & Marcy," he appears during a flashback from 996 years ago, proving that he has has a lifespan much larger than that of a normal snail. However, this could possibly just be another snail entirely.
 * In "Evicted!" during the "House Hunting Song," the snail appears twice in one sequence, when Finn and Jake are walking up a hill. This has not happened again after this episode.
 * In other TV shows, things like baby heads serve a similar theme of being a creepy easter egg lurking in the background of multiple episodes with sinister implications.
 * In The Adventure Time Encyclopædia, Marceline states that the Snail, along with many other characters, lives in the Grass Lands.
 * The snail was revealed to work in a traveling circus in "Sad Face," where he does an act where he jumps into a bucket.
 * He does not appear in "Food Chain", due to the fact that the episode was made by a guest animator.

Appearance in other media

 * In the game Righteous Quest 2, the Snail appears a few times in the game.
 * The Snail starred in its own FusionFall mission: "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 cameos 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.
 * The snail appears at random points in Finn and Jake's Epic Quest and is sometimes the objective of a certain quest.
 * The snail was the only main character that didn't wear a sweater in Holly Jolly Secrets Part II.
 * The Snail appears in the Adventure Time graphic novel, Playing with Fire.
 * The Snail appears in Finn's backpack on the Adventure Time Thanksgiving Day Parade balloon.
 * A revolving 3D model of the Snail is used as a loading icon in the Adventure Time iOS game, Card Wars. Specifically it can be seen on the top-left corner of the screen when starting up the app.
 * The snail can be seen on the blue team's side in Adventure Time Battle Party.
 * In Finn & Bones, the snail can be seen on the arrow in the last picture of the game's ending, when Death does the "Kiss of Death" to Jake.

References in other media

 * In Issue 520 in MAD magazine, the snail can be seen on the cover, waving. Although, the snail is waving angrily at the reader.