Therianthropy refers to the metamorphosis of humans into other animals. The term therianthropy comes from n. therianthrope and adj. therianthropic, part man and part beast, from the Greek theríon, θηρίον, meaning "wild animal" or "beast" (impliedly mammalian), and anthrōpos, άνθρωπος, meaning "human being". It was used to refer to animal transformation folklore of Asia and Europe as early as 1901. Sometimes, "zoanthropy" is used instead of "therianthropy."some related words from the OED:
Theriocephaly is the condition or quality of having the head of an animal - commonly used to refer the depiction in art of humans (or deities) with animal heads. [examples at the link]
Theriac is an antidote to poison, esp poison from a serpent. Apparently the flesh of a serpent was an integral part of the antidote, and thus the derivation of the word.
Theriodont = fossil reptile with mammalian teeth.
Theriolatry = worship of beasts
Theriomancy = divination using animals
Theriopod = having the feet of a beast
Theriozoic = re the time in human history before domestication of animals
Theriomaniac = one with a mania for hunting wild animals
Addendum: Paul points out that the phrase To Mega Therion (Τὸ Μεγα Θηρίον) is Greek for "The Great Beast" (from the Book of Revelation: "And I stood upon the sand of the sea, and saw a beast [θηρίον, therion] rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy. 2 And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority." (Revelation 13:1) and has been used to refer to Aleister Crowley.
?wasn't there a megatherium in the Hitchhiker's Guide to the Galaxy?