Now onto shapeshifting or shapeshifters.

In mythology, folklore and speculative fiction, shape-shifting is the ability to physically transform oneself through an inherently superhuman ability, divine intervention, demonic manipulation, sorcery, spells or having inherited the ability. The idea of shape-shifting is in the oldest forms of totemism and shamanism, as well as the oldest existent literature and epic poems such as the Epic of Gilgamesh and the Iliad. The concept remains a common literary device in modern fantasy, children's literature and popular culture.

Popular shape-shifting creatures in folklore are werewolves and vampires (mostly of European, Canadian, and Native American/early American origin), ichchadhari naag and ichchadhari naagin (shape-shifting cobras) of India, the huli jing of East Asia (including the Japanese kitsune and Korean kumiho), and the gods, goddesses, and demons and demonesses like succubus and incubus and other numerous mythologies, such as the Norse Loki or the Greek Proteus. Shape-shifting to the form of a gray wolf is specifically known as lycanthropy, and such creatures who undergo such change are called lycanthropes. Therianthropy is the more general term for human-animal shifts, but it is rarely used in that capacity. It was also common for deities to transform mortals into animals and plants.

Other terms for shapeshifters include metamorph, the Navajo skin-walker, mimic, and therianthrope. The prefix "were-", coming from the Old English word for "man" (masculine rather than generic), is also used to designate shapeshifters; despite its root, it is used to indicate female shapeshifters as well.

While the popular idea of a shapeshifter is of a human being who turns into something else, there are numerous stories about animals that can transform themselves as well.

Greco-Roman

Examples of shape-shifting in classical literature include many examples in Ovid's Metamorphoses, Circe's transforming of Odysseus' men to pigs in Homer's The Odyssey, and Apuleius's Lucius becoming a donkey in The Golden Ass. Proteus was noted among the gods for his shape-shifting; both Menelaus and Aristaeus seized him to win information from him, and succeeded only because they held on during his various changes. Nereus told Heracles where to find the Apples of the Hesperides for the same reason.

The Oceanid 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 who 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 Greek mythology, the transformation is often a punishment from the gods to humans who crossed them.

Zeus transformed King Lycaon and his children into wolves (hence lycanthropy) as a punishment for either killing Zeus' children or serving him the flesh of Lycaon's own murdered son Nyctimus, depending on the exact version of the myth.

Ares assigned Alectryon to keep watch for Helios the sun god during his affair with Aphrodite, but Alectryon fell asleep, leading to their discovery and humiliation that morning. Ares turned Alectryon into a rooster, which always crows to signal the morning and the arrival of the sun.
Demeter transformed Ascalabus into a lizard for mocking her sorrow and thirst during her search for her daughter Persephone. She also turned King Lyncus into a lynx for trying to murder her prophet Triptolemus.

Athena transformed Arachne into a spider for challenging her as a weaver and/or weaving a tapestry that insulted the gods. She also turned Nyctimene into an owl, though in this case it was an act of mercy, as the girl wished to hide from the daylight out of shame from being raped by her father.

Artemis transformed Actaeon into a stag for spying on her bathing, and he was later devoured by his own hunting dogs.

Galanthis was transformed into a weasel or cat after interfering in Hera's plans to hinder the birth of Heracles.

Atalanta and Hippomenes were turned into lions after making love in a temple dedicated to Zeus or Cybele.

Io was a priestess of Hera in Argos, a nymph who was raped by Zeus, who changed her into a heifer to escape detection.
Hera punished young Tiresias by transforming him into a woman and, seven years later, back into a man.

King Tereus, his wife Procne and her sister Philomela were all turned into birds (a hoopoe, a swallow and a nightingale respectively), after Tereus raped Philomela and cut out her tongue, and in revenge she and Procne served him the flesh of his murdered son Itys (who in some variants is resurrected as a goldfinch).
Callisto was turned into a bear by either Artemis or Hera for being impregnated by Zeus.
Selene transformed Myia into a fly when she became a rival for the love of Endymion.
While the Greek gods could use transformation punitively – such as Medusa, turned to a monster for having sexual intercourse (raped in Ovid's version) with Poseidon in Athena's temple – even more frequently, the tales using it are of amorous adventure. Zeus repeatedly transformed himself to approach mortals as a means of gaining access:

Danaë as a shower of gold
Europa as a bull
Leda as a swan
Ganymede, as an eagle
Alcmene as her husband Amphitryon
Hera as a cuckoo
Aegina as an eagle or a flame
Persephone as a serpent
Io, as a cloud
Callisto as either Artemis or Apollo
Nemesis (Goddess of retribution) transformed into a goose to escape Zeus' advances, but he turned into a swan. She later bore the egg in which Helen of Troy was found.