The preferred terms in this encyclopedia for those who change shape (and for the act of thus changing shape) repeatably and reversibly, by innate Magic, Talent or breeding. This is distinct from Metamorphosis, whose tendency is to be radical, unique and permanent, and Transformation, which is generally imposed by an outside magical agency, as with the Frog Prince.
Fantasy's best-known shapeshifters are the were-creatures or were-people (>>> Theriomorphy), the commonest being Werewolves – a trope of Horror because so often the shift is not voluntary but seen as a Curse, a Bondage to the full Moon which brings on wolf-shape and/or a Possession by a wolf's Spirit. The lighter fantasy favoured by Unknown took a cheerier view of such shapeshifting and, typically ringing the changes, discussed a wider selection of target animals. Anthony Boucher's "The Compleat Werewolf" (1942) and Poul Anderson's "Operation Afreet" (1956) both accentuate the positive with heroes who find wolf-form useful when fighting, while in quieter times it offers a movie stunt-dog career; both consider other were-forms, including tigers – Anderson also offers a were-fennec or desert fox, while Boucher's broad Humour expands from bears (Beorn in J R R Tolkien's The Hobbit  is a were-bear) to a were-ant and a were-diplodocus. Jack Williamson's altogether grimmer Darker Than You Think (1948) likewise enlarges the range of weredom with a pterosaur form. More traditional shapeshifters are the Chinese fox women, the Scots Selkies or seal men, and Vampires – whose subsidiary ability to shapeshift into bat form seems to have fed back into fiction after the vampire bat was so named in the 18th century. Shifting to Serpent form is generally reserved for women of bad character (> Lamia), like the witch in C S Lewis's The Silver Chair (1953).
Further examples are too numerous to list. Variations include shapeshifters whose default form is beast rather than human – like the were-human wolf in Larry Niven's "What Good is a Glass Dagger?" (1972) and the Cat Greebo in Terry Pratchett's Discworld sequence, who after being briefly transformed to human form learns the knack and can repeat the effect by shapeshifting. The heroine of Piers Anthony's A Spell for Chameleon (1977) shapeshifts gradually and repeatedly between the poles of moronic beauty and highly intelligent ugliness; the hero decides this is a good thing since he will never become bored with her (> Feminism).
Fairies, Witches and Wizards, if they shapeshift at all, are not confined like were-creatures to a single alternative form; the shapeshifter villains of Patricia McKillip's Riddle-Master trilogy can be pretty much what they wish. One traditional form of magical duel has the antagonists flashing from shape to shape in search of advantage: the solitary highlight of the movie The Sword in the Stone (1963; > T H White) is such a contest between Merlin and "Madam Mim", who loses when polymath Merlin finally becomes a bacillus and infects her. Ursula K Le Guin, ever concerned with Balance, presents a darker side of shapeshifting in A Wizard of Earthsea (1968), where the temptation of another form can lead to loss of humanity as the animal body reshapes the Soul within: her hero nearly loses himself when fleeing in hawk shape. Barbara Hambly offers a subtler version of this lure in Dragonsbane (1986), where becoming a Dragon represents a clear improvement on being human. Voluntary shapeshifting offers broad vistas of wish-fulfilment, but may be a Thing Bought at Too High a Cost if it should become fixed as permanent Metamorphosis. [DRL]