Origins[]
The Wendigo is a creature with origins in Native American myth and legend, specifically among the Algonquian people (and other Algonquian speaking tribes).
It is a malevolent spirit associated with gluttony, cannibalism and similar unpleasant conditions.
It poses a threat to all who encounter it, but those in particular danger of being taken over by the spirit's influence were those who were starving, greedy, gluttonous or who themselves, practiced cannibalism.
Its connection to the practice of cannibalism and famine led tribe elders to regard cannibalism as highly taboo, even in times of great hunger and famine within the tribe.
Not only could it draw a Wendigo's attention to one's tribe, but it could even result in a person turning into a Wendigo.
Appearance[]
The appearance of the Wendigo is widely varied.
For example, an Ojibwa teacher and scholar from Ontario, Basil Johnston, described the Wendi go as follows:

"MUST...FEED..."
"The Wendie was gaunt to the point of emaciation, its desiccated skin pulled tautly over its bones. With its bones pushing out against its skin, its complexion the ash gray of death, and its eyes pushed back deep into their sockets, the Wendie looked like a gaunt skeleton recently disinterred from the grave. What lips it had were tattered and bloody [....] Unclean and suffering from suppuration of the flesh, the Wendie gave off a strange and eerie odor of decay and decomposition, of death and corruption."
Though, others accounts say that its appearance is similar to a vampire, a werewolf, a zombie, a Sasquatch, a demon or some terrifying abomination combining any of the former.
And some are mortal, while others are spirit. Those that are in a spirit form are even harder to identify, since people often confuse their incorporeal forms with run-of-the-mill ghosts.
Thus, many suppose that the creatures have no distinct form, but only an amorphous shapeshifting elemental with a connection to the forces of cold, winter, hunger and gluttony. This theory is widely accepted.

"She looks good enough to eat!"
Behavior[]
The Wendigo can be described by few other words besides malevolent and ravenous. It is unrelenting and uncaring of age, race, gender, religion or sexual orientation. It desires only to eat, to control and to kill.
It eats all, attacks any it desires, and will not stop, as it is never satisfied. Though, due to the means by which it is connected to the afterlife and the spirit world, it is most often found in colder regions.
However, this affinity for the cold does not preclude attacks in temperate or warmer regions. Its hunger takes it wherever it senses more food, or more potential victims of its possession.
Now, one would ask, if a Wendigo can interact with the physical world without a physical body, why would it want or need to take possession of a mortal form? The answer? It doesn't. Rather, it simply delights in the pain that possession causes, and in turning people against one another; one of its numerous sadistic pleasures...
Abilities[]

The ravenous Wendigo will NEVER be satisfied...
It is the embodiment of cold, winter, famine (especially winter famine), excess, greed, gluttony and cannibalism. As such, it is a being of great and frightening spiritual power. Its aura, uncloaked, is significant to frighten away most, whether animal, humanoid or human.
In addition, the connection of the Wendigo to winter gives it the ability to withstand the cold entirely, while its supernatural predatory nature grants it enhanced strength, speed, endurance, senses, reflexes and awareness (with fierce fangs and vicious claws to boot).
Not to mention, though capable of interacting with the physical world (similar to a poltergeist), a Wendigo exists naturally in a spirit form, which is capable of the typical ghostly abilities; invisibility, intangibility, inaudibility, levitation, flight, etc. Even possession.
However, it typically saves the latter for more particular prey; those who live in colder areas, are starving, are gluttonous, are greedy, are particularly extravagant in displaying wealth or are cannibals.
Those who become possessed by a Wendigo are considered to be Wendigos themselves, and they will remain so until either the Wendigo leaves, it is exorcised, or the possessed victim is killed (the former two are most likely).