Wendigo
Reason number 1049030 why to stay the hell out of woods!
The Wendigo is a supernatural spirit that possesses those who commit cannibalism; they begin to mutate into something more akin to a deer, not a living deer, a rotting corpse of a deer. Now horribly mutated, their hunger for man buns is increased tenfold and they are bewitched with supernatural powers. They are a Demon/Cryptid from Midwestern Native American tribes, but the mythology is also native to the northern forests of the Atlantic Coast and Great Lakes Region of both the United States and Canada.
Folklore
The Wendigo is a creature of the winter famines; those who succumb to desperation in the face of hunger and feed themselves by dining on the flesh of their own kind are possessed by an evil spirit that kills them and twists their body into a new form. For obvious reasons, the Wendigo are associated with greed, cannibalism, murder, and cultural taboo.
Folklore often serves to enforce a practical lesson, and with the Wendigo, that's obvious: don't eat other people! Besides that being incredibly antisocial, there's also legitimate health reasons to do so, as at least one kind of prion disease is transmitted through this kind of act -- basically, if you eat somebody's brain, your own brain may begin rotting as infectious prions convert your neural tissue into yet more infectious prions.
Appearance
Wendigos are given different appearances in different regions, although the general nastiness remains pretty consistent. One of the more iconic versions is a emaciated, skeletal, man-deer monster, often with a naked skull for a head. Other versions include creepy giant ghoul-like monsters and frostbitten giants, sometimes with the original human form visible inside the chest (rather like the Dungeons & Dragons Devourer, actually). All Wendigos, however, are described as being enormous, towering above ordinary people. They're often described as being footless as well; their supernatural speed turning their feet into little more than bloody stumps.
Powers
Wendigos are associated with winter, and often said to be able to call forth massive blizzards and howling gales, making winter far nastier. Other common tricks include teleportation and/or moving faster than the eye can see, voice mimicry, raking claws that can rend men apart or pull out their hearts, a freezing-cold touch that lets them kill without using their claws, and the ability to induce madness and cannibalistic hunger in others.
Most tribes consider even speaking their names to be taboo, especially during winter.
Another part of a story is that if you hear your voice being called forth from the woods, you need to have your friend or family member tie you down, Chances are the wendigo will leave you alive but with a sore asshole.
/tg/ Relevance
H.P. Lovecraft adapted the Wendigo into his mythos, saying such myths are either misremembered encounters with the Great Old One Ithaqua, or tell of victims transformed by his malign influence. Ithaqua has appeared in multiple editions of Call of Cthulhu.
Pathfinder, given its willingness to adopt non-European mythical beasts into its ever-growing arsenal of beasties, naturally features Wendigo as one of its more prominent - and powerful! - monsters.
In Werewolf: The Apocalypse, the Wendigo are one of the Garou tribes. They were chill (no pun intended) Natives who got screwed over by the European Garou and are still salty about it to this day.
The day in which a Neckbeard Encounters one, though this will likely result in the neckbeard being seen as a tasty treat and getting Murderfucked out of existence, because cmon when was the last time you saw a fat fuck outrun a supernatural man eating beast, never. Though the unrealistic part of this is a neckbeard being outside let alone in the woods.