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.

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, and at the end of "In Your Footsteps" with his first lines.

"In Your Footsteps"
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. This has stunted the mystery whether or not the Lich has a plan to have revenge on Finn for slaying him (or attempting to, since the Lich is still living on through the Snail). 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

 * In Righteous Quest 2, the Snail appears a couple times in the game.
 * The Snail starred in its own FusionFall game: "Snail Quest."
 * 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, and it even shares its form with the hidden snail of Blue's Clues.
 * The Adventure Time game Legends of Ooo uses snails as hints. Also, snail is used to free Slime Princess.
 * "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. 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," "Adventure Time with Fionna and Cake," and "What Was Missing").
 * The Snail was given a new, greener design. Its only been shown in "Apple Thief" episode, but this may be just because of lighting.
 * "Holly Jolly Secrets Part Two," "Ghost Princess," and "Dad's Dungeon" all have had the snail in the preview.
 * The Snail's eyes are below where an actual snail's eyes are.
 * The Adventure Time Comic series begins with the Lich-controlled Snail releasing The Lich after being defeated by Finn.
 * 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, only defeated by Finn and Jake.
 * The Snail has appeared in nearly 97.4% of Adventure Time episodes.
 * In "His Hero," and "Wizard Battle" only his shell is shown.
 * In "It Came from the Nightosphere," its soul looks like its physical body (not like the other ones that look like regular ghosts).
 * The Snail appeared in the title card of "Mortal Recoil".
 * 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.
 * It is unknown how the Snail got to the Nightosphere in "Return to the Nightosphere".
 * It is possible the snail sometimes travels in Finn's pack (as seen in "Mortal Folly").
 * The Snail is voiced by Pendleton Ward.