They are a group of Satan-worshiping woodland animals created by Cartman in a Christmas story for a class assignment. Though they appear friendly, they can manifest "evil Satanic powers", such as the ability to conjure fire. They originally appeared in "Woodland Critter Christmas", in which they attempted to bring about the birth of the Anti-Christ so that Satan can rule the world, but are ultimately killed by Santa Claus when he shoots them with a shotgun. However, this story in itself was an "imaginary" one, and did not actually occur in the "real world" of the story.
The twelve critters are: Squirrely the Squirrel, Rabbity the Rabbit, Beavery the Beaver, Beary the Bear, Porcupiney the Porcupine, Skunky the Skunk, Foxy the Fox, Deery the Deer, Woodpeckery the Woodpecker, Mousey the Mouse, Raccoony the Raccoon and Chickadee-y the Chickadee.
Most of the critters also have their own catchphrase or verbal tic; Squirrely's "now don't be down, y'all", Mousey's "I deduce", Beary's "but we got to -" and Chickadee-y's repeating of the last few words of her statements.
In the eleventh season story arc "Imaginationland", they appeared in reality. Because they were made up by Cartman they actually exist in the evil realm of Imaginationland, and were freed when terrorists destroyed the barrier separating the benevolent imaginary characters from the evil ones. In one scene, in which other famous villains such as Freddy Krueger and Jason Voorhees argue as to what to do with a captured Strawberry Shortcake, the Critters actually disturb the other villains with their grotesque suggestions on how to mutilate, torture and kill Shortcake, with Jason Voorhees saying, "Man, I do not want to meet the kid that dreamt those things up." The Critters also raped (and presumably killed) a squad of American soldiers led by Kurt Russell that crossed into Imaginationland. They managed to make a Tron Soldier's head explode, they also attacked and gangraped Popeye in the final battle, but did not manage to kill him. They were eventually all put back behind the barrier to Evil Imaginationland along with the rest of the evil imaginary characters after Butters used his imagination to restore Imaginationland back to the way it was before the terrorist attack.
The Woodland Critters are deceptively cute. They all have bulgy, shiny eyes. They also wear either colorful scarves or hats.
They act strangely optimistic and childlike, despite being evil. They also seem to enjoy guro, evidenced by the fact that they had a blood orgy after sacrificing Rabbity the Rabbit in "Woodland Critter Christmas". They also seem enjoy seeing others suffer, they wanted to torture Strawberry Shortcake before they killed her in Imaginationland.
They originally befriended Stan, but only to manipulate him into killing the Mountain Lion Cubs' mother. After Stan realized they were Satan-worshippers, he tried to stop them, leading to them becoming enemies.
The Woodland Critters appear as enemies in South Park Let's Go Tower Defense Play!. The critters are faster than the other enemies of the game and tend to deal between nine and eleven damage to players' towers. They are weak against fiery-works towers.
The Woodland Critters appear as enemies in South Park: Tenorman's Revenge. The appear during the Hell level.
The Woodland Critters appear as non-player characters in South Park: The Stick of Truth. They can be found in a clearing in the Lost Forest next to a large Christmas tree, and ask the player to accept Satan as their one true lord. When the player does, all 12 critters friend him on Facebook.