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"). It is seen making cameos, usually waving at the screen. Its cameos are a running gag.

The Lich
While the Snail was just a running gag at first, it 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. It appears again at the end of "Mortal Recoil" still being controlled by the Lich.

For the first few episodes of season three, the Snail continues to appear in each episode, and (when examined closely) appears to still have green eyes and an evil expression on its face.

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

Season Three
In season three, the snail is still possesed by The Lich.

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 in My Little Pony: Friendship is Magic, and it even shares it's form with the hidden snail of Blue's Clues.
 * Pendelton Ward refers the Snail to be an "Easter egg".
 * The new Adventure Time game Legends of Ooo uses snails as hints. Also, a snail is used to free Slime Princess.
 * The Four Episodes, "Mortal Folly," "Recoil," "Storytelling," and "Prisoners of Love" were the only Four times where viewers could easily notice the Snail.
 * Multiple times after 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. It hasn't, and probably will never, appear in the show.
 * 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.
 * It has been rumored the Snail is trying to kill Finn and Jake, because on most of Season 3 episodes he is spotted near a weapon, although this is not confirmed.
 * Finn is the only one (not counting The Lich) to have seen and or spoken about the Snail, as of Storytelling.