Shapeshifters

Note About Shapeshifters
The idea of shapeshifting covers a range of categories including shapeshifters, skin-walkers, doppelgängers, and changelings (amoung others, including werewolves). A few resources will be given on the broad range of categories.

Excerpt from Wikipedia about the Purpose of Changelings
Some people believed that trolls would take unbaptized children. Once children had been baptized and therefore become part of the Church, the trolls could not take them. One belief is that trolls thought that being raised by humans was something very classy, and that they therefore wanted to give their own children a human upbringing.

Beauty in human children and young women, particularly blond hair, attracted the fairies.[4]

In Scottish folklore, the children might be replacements for fairy children in the tithe to Hell;[5] this is best known from the ballad of Tam Lin.[6] Also, according to common Scottish myths, a child born with a caul (head helmet) across their face is a changeling, and of fey birth.

Some folklorists believe that fairies were memories of inhabitants of various regions in Europe who had been driven into hiding by invaders. They held that changelings had actually occurred; the hiding people would exchange their own sickly children for the healthy children of the invaders.[7]

In other folklore, the changelings are put in place of the child to feed off of the mother of the child. The kidnapped child then becomes food for the changeling's mother. This is done for the survival of their kind. Once the changeling mother and the changeling have drained the life from the human mother and child, the changeling and its mother begin to search for a new suitable food source. Other sources[2] say that human milk is necessary for fairy children to survive. In these cases either the newborn human child would be switched with a fairy babe to be suckled by the human mother, or the human mother would be taken back to the fairy world to breastfeed the fairy babies. It is also thought that human midwives were necessary to bring fairy babes into the world.

Some changelings might forget they are not human and proceed to live a human life. Changelings which do not forget, however, may later return to their fairy family, possibly leaving the human family without warning. As for the human child that was taken, he or she may often stay with the fairy family forever.

Excerpt from TVTropes about Shapeshifting
Shapeshifting and transformation are the ability to completely change one's body from one form to another, usually by magic or Applied Phlebotinum. Common in Speculative Fiction, and folklore and myths are replete with tales of princes transformed into frogs, gods turning themselves into animals, and so on. Several of these tropes are Older Than Feudalism.

There are a wide variety of ways that this can occur, but thematically it can be sorted into two broad classes:
 * Shapeshifting: A character can transform their own body as a special power or ability. Obviously enough, they are called shapeshifters, and often ply their powers for fun and profit (whether good or evil), or simply to trick or play practical jokes on normal folks.
 * Transformation: An otherwise non-shapeshifting character has a transformation cast upon them from an outside source (again, usually by magic or Applied Phlebotinum). This is typically Played for Drama; it may be part of a character's backstory, or even form the central conflict within the work. Curses are a popular source here, as it requires the character to go on a Quest to satisfy the Curse Escape Clause. A list of possible causes for transformations can be found here.

Excerpt from Wikipedia about Shapeshifting Powers
One motif is a shape change in order to obtain abilities in the new form. Berserkers were held to change into wolves and bears in order to fight more effectively. In many cultures, evil magicians could transform into animal shapes and thus skulk about.

In many fairy tales, the hero's talking animal helper proves to be a shapeshifted human being, able to help him in its animal form. In one variation, featured in The Three Enchanted Princes and The Death of Koschei the Deathless, the hero's three sisters have been married to animals. These prove to be shape-shifted men, who aid their brother-in-law in a variant of tale types.[22]

In Greek mythology, the Titan Metis, the first wife of Zeus and the mother of the goddess Athena, was believed to be able to change her appearance into anything she wanted. In one story, she was so proud, that her husband, Zeus, tricked her into changing into a fly. He then swallowed her because he feared that he and Metis would have a son that would be more powerful than Zeus himself. Metis, however, was already pregnant. She stayed alive inside his head and built armor for her daughter. The banging of her metalworking made Zeus have a headache, so Hephaestus clove his head with an axe. Athena sprang from her father's head, fully grown, and in battle armor.

In an early Mayan text, the Shapeshifter, or Mestaclocan, has the ability to change his appearance and to manipulate the minds of animals. In one tale, the Mestaclocan finds a dying eagle. Changing into the form of an eagle, he convinces the dying bird that it is, in fact, not dying. As the story goes they both soar into the heavens, and lived together for eternity.

This use, though rare in older fiction, is perhaps the most common in modern fiction. Several superheroes and supervillains — such as Beast Boy, Chameleon Boy/Chameleon, Morph, Ben Tennyson, Mystique and Clayface — have it as their sole power. The Harry Potter series contains both Animagi who can change to a single animal form and Metamorphmagi who can alter their appearance. The Twilight Saga also features shapeshifters that can transform into wolves and have inhuman strength, speed, body temperature and aging process.[23] Several episodes of the television shows: True Blood and Supernatural featured shape-shifters. Both the Earthmasters and their opponents in The Riddle-Master of Hed trilogy make extensive use of their shape-shifting abilities for the powers of their new forms.[24]

Resources
Wikipedia article on Shapeshifting

Lost Tapes: Skinwalkers (Youtube)

Wikipedia article on Changelings

TVTropes article on Shapeshifting