Skin Thieves are one of the many, many, many horrible paranoia-inducing monsters native to the Demiplane of Dread, which DMs are encouraged to use to spook the living hell out of any Dungeons & Dragons players foolish enough to agree to a game of Ravenloft.
As their name implies, skin thieves are a more visceral equivalent to the common Doppelganger. In their native form, skin thieves resemble lanky humanoid bears, with overly long, ape-like arms that end in eight-fingered hands, with each finger tipped by a dagger-like claw ranging from 8 to 16 inches in length - sort of a more monstrous/furry version of a bugbear. A skin thief's talons are its pride and joy; they love to decorate their claws, and they crave jewelry in the form of rings, bracers, and any other items that draw attention to their long nails. Their moniker comes because they have the ability to painstakingly peel off humanoid skins and (somehow) cram themselves inside the flayed hides, allowing them to disguise themselves as normal people. The hide is fragile, and will usually rip apart in the heat of battle to gruesomely reveal the beastman inside, but it can make a surprisingly convincing disguise to let the skin thief get closer to its victim.
As a species, skin thieves are a race of parasites. They have no true culture of their own, and prefer to steal property rather than make it; they don't even have a natural language of their own, instead speaking make-shift pidgin tongues made up of the various humanoid languages they have overheard. They pursue a nomadic existence, typically hovering on the outskirts of civilization - trade routes and areas of sparse habitation are preferred, as this gives them plenty of cover to kill humanoid travelers for food, loot and disguises and get away with it. They usually shelter in caves or small, crudely built lean-tos for periods of up to a month, but when they succeed in wiping out a small caravan or farmstead, they will typically wear their victim's hides and take up living there in a twisted parody of the human lifestyle. Such disguised skin thieves will even attempt to interact with other humans under a semblance of normality, mostly to catch new victims; their behavior and crude speech patterns invariably give them away.
Skin thieves live in clans of two to three family units, lead by a "chief" who attains power through his or her ability to intimidate the others into obedience. For obvious reasons, they rarely stay in power for more than a few months, and the leadership of a skin thief clan is almost always in flux. Nobody knows where these twisted, vicious, petty creatures came from; the dominant theory amongst the scholars of the Demiplane of Dread is that they descend from clans of human bandits who turned to cannibalism.