Wendigo

Origin: It has many origins and legends but the first legend was started by the Algonquian, a native american tribe.

Summary and or explanation: (This one's a doozy folks, it's also one of my favorite mythological monsters.)

The Wendigo is a half-beast creature appearing in the  of the  along the  Great Lakes Region of both the United States and Canada . The creature or spirit could either possess characteristics of a human or a monster that had physically transformed from a person. It is particularly associated with cannablism. The Algonquian believed those who indulged in eating human flesh were at particular risk; the legend appears to have reinforced the  of the practice of cannibalism. It is often described in Algonquian mythology as a balance of nature.

The legend lends its name to the disputed modern medical term Wendigo psychosis. This is supposed to be a  that features symptoms such as an intense craving for human flesh and a fear the sufferer is a cannibal. (Back then they didn't know about something called Pica.) This condition was alleged to have occurred among Algonquian native cultures, but remains disputed.

I'm going to use the first legend told by the Algonquian people, because many people and documents say that the Wendigo originated from their mythology. 

The Wendigo is part of the traditional belief systems of various Buffalo people in the northern United States and Canada, most notably the Ojibewe and Saulteaux, the Cree, the Naskapi, and the Inuu People.  Although descriptions varied somewhat, common to all these cultures was the conception of Wendigos as beings of great spiritual power.

They were strongly associated with the winter, the north, and coldness, as well as with  and starvation.  (Which is why only fire can destroy them or in the case of my book, grenades.)

At the same time, Wendigos were embodiments of gluttony, greed, and excess: never satisfied after killing and consuming one person, they were constantly searching for new victims. In some traditions, humans who became overpowered by greed could turn into Wendigos; the Wendigo myth thus served as a method of encouraging cooperation and moderation.

Among the Ojibwe, Eastern Cree, Westmain Swampy Cree, Naskapi , and Innu, Wendigos were said to be giants, many times larger than human beings (a characteristic absent from the Wendigo myth in the other Algonquian cultures). Whenever a Wendigo ate another person, it would grow in proportion to the meal it had just eaten, so that it could never be full.  Therefore, Wendigos were portrayed as simultaneously gluttonous and emaciated from starvation.

All cultures in which the Wendigo myth appeared shared the belief that human beings could turn into Wendigos if they ever resorted to cannibalism, or, alternatively, become possessed by the demonic spirit of a Wendigo, often in a dream. Once transformed, a person would become violent and obsessed with eating human flesh. The most frequent cause of transformation into a Wendigo was if a person had resorted to cannibalism, consuming the body of another human in order to keep from starving to death during a time of extreme hardship, for example in hard winters, or famine.

Among northern Algonquian cultures, cannibalism, even to save one's own life, was viewed as a serious ; the proper response to famine was  or resignation to death. On one level, the Wendigo myth thus worked as a deterrent and a warning against resorting to cannibalism; those who did would become wendigo monsters themselves.

Among the Assiniboine, the Cree and the Ojibewe , a  ceremonial dance originally was performed during times of famine to reinforce the seriousness of the Wendigo . The ceremonial dance, known as a wiindigookaanzhimowin in Ojibewe  and today performed as part of the last day activities of the Sun Dance, involves wearing a mask and dancing about the drum backward. The last known wendigo ceremony conducted in the United States was at the Star Island of Cass Lake, located within the  in Leech Lake Indian Reservation in northern Minnesota .

(So don't eat anyone and you won't change into a monster, simple as that. A lot of the really deep stuff came from Wikipedia while I do know a lot about mythology I don't know everything.)