In Navajo (Navajo: Diné) culture, a skin-walker (yee nahgloshii) is a type of witch who has the ability to turn into an animal, or to disguise themselves as an animal. Typically, a skin-walker will take on the form of a coyote, wolf, owl, fox, or crow, though they possess the ability to take on any form they desire.
In the Navajo language, yee naagloshii translates to "he who walks on all fours". While perhaps the most common variety seen in horror fiction by non-Navajo people, the yee naaldlooshii is one of several varieties of Navajo witch, specifically a type of ’ánti’įhnii. The legend of the skin-walkers in uncertain, mostly due to outsiders not being allowed in on this subject, thus people are led to draw their own conclusions from the stories they hear.  According to the folklore archives in BYU's World History & Culture, an article about skin-walkers states that, "The fact that there aren't many stories of skin-walkers using their power to harm doesn't mean there aren't a few. The important thing to remember about skin-walkers is that they are human.  With this idea in mind, it becomes hard to know what is fact and what is fiction, along with realization that not being part of the Navajo culture, it is near impossible to fully understand the legend of the skin-walker.
Navajo people usually hesitate to reveal this lore to non-Navajos, or even to discuss it casually among those they do not trust. People outside the Native American loop are scarcely allowed to know information surrounding skin-walkers and speaking of these witches is considered a taboo in the culture, though it is not entirely clear as to why. In some cases it is believed that by talking about the yee nahgloshii, it will gain their attention which can be dangerous and can lead to people being targeted.
Difference from medicine man
In the Navajo culture there is a clear distinction between a witch and a medicine man. Medicine men practice healing arts, blessings, and the removal of curses. Any Navajo practicing the witchery way is believed to be evil; the intent of such practice is purely to harm others. Skinwalkers are considered to once have been medicine men who were able to reach the highest level of priesthood. These healers, instead of using their abilities to help people, would use their power for works of evil and take on animal form, inflicting pain on others.
There have been several eyewitness accounts claiming to have encountered or at least seen a skinwalker. Several of these reports have come from missionaries who have seen someone, adorned in animal skin, running alongside their car while heading down an open highway, before darting off into the surrounding wilderness. 
Various skin-walker tales can be found scattered over the web. Whether they're hoax or truth is unknown, yet there are some common similarities throughout these sightings. Apart from claims that these "creatures" can run at ridiculously high speeds, they are also said to have a human feature when they are in animal form. Some have been claimed to walk standing on their hind legs. Other cases have stated that skin-walkers walk in the same manner as a person crawling on all fours, rather than a typical four-legged animal. The face happens to be another physical feature that appears human on these animals. The face itself has been depicted as being human, just pasted onto the body of an animal. The eyes are another telltale sign, being described as being too big, like those of a human, where you can see the whites of their eyes. Lastly, skin-walkers are said to have no tail when in animal form. Dogs are supposedly able to detect skin-walkers, presumably from their unique difference from other animals. 
Many accounts of skin-walkers show them to be eerie and mysterious, however there are very little accounts of the yee nahgloshii actually killing anyone. Skin-walkers, tend to just scare people lingering outside their windows, knocking on the walls, running alongside cars, and simply startling people with their presence. 
- Wall, Leon and William Morgan, Navajo-English Dictionary. Hippocrene Books, New York City, 1998 ISBN 0-7818-0247-4.
- , Hampton, Carol M., "'Some Kind of Power: Navajo Children's Skinwalker Narratives'/Western Historical Quarterly." 01 July 1986. Accessed 17 Nov. 2016.
- , "Skin-Walkers (Navajo Legend)." Harold B. Lee Library/World History & Culture, BYU, 3 Sept. 2014. Web accessed 18 Nov. 2016.
- Keene, Dr. Adrienne, "Magic in North America Part 1: Ugh." at Native Appropriations, 8 March 2016. Accessed 9 April 2016. "What happens when Rowling pulls this in, is we as Native people are now opened up to a barrage of questions about these beliefs and traditions…but these are not things that need or should be discussed by outsiders. At all. I'm sorry if that seems 'unfair,' but that's how our cultures survive."
- Brady, Margaret (1984). "Some Kind of Power": Navajo children's skinwalker narratives. University of Utah Press.
- Morgan, William (1936). "Human-Wolves among the Navaho". Yale University Publications in Anthropology. 11.
- Salzman, Michael (October 1990). "The Construction of an Intercultural Sensitizer Training Non-Navajo Personnel". Journal of American Indian Education. 30 (1): 25–36.
- Walsh, Patrick (Spring 1974). "The Skinwalker". Affword. 1.