Trans-Species Transformation

Cross-Species Transfiguration is a branch of Transfiguration in which one animal is Transfigured into another. It is a basic O.W.L. spell and is taught to fourth-years and above at Hogwarts School of Witchcraft and Wizardry. The spell seems to be easier if the animals in question have similar names, e.g. guinea fowl to guinea pigs.

Although humans are creatures too, it is harder to Transfigure a human into an animal, especially self-Transfiguration, and so this is not taught until sixth-year. There is a distinct difference between Transfiguring oneself into an animal (which requires a wand), and transforming into an animal at will (which doesn't require a wand).

Known instances of Cross-Species Transfiguration

 * Circe, a witch who lived in ancient times, was famous for turning sailors into pigs.
 * In 1991, Hagrid attempts to turn Dudley into a pig, however, only gives him a pig's tail.
 * A man stated that he was able to turn his wife into a yak after he took a Kwikspell course.
 * In 1992, a student accidently changed his classmate into a badger.
 * In 1995, Draco Malfoy was Transfigured into a ferret by Alastor Moody as punishment for attempted to attack Harry while his back was turned.
 * In 1995, fourth-year students Transfigure guinea fowl into guinea pigs.
 * While competing in the second task of the Triwizard Tournament in 1996, Viktor Krum attempts to Transfigure himself into a shark, however, he only gave himself a shark's head.
 * In 1996, Hannah Abbott accidently Transfigured her ferret into a flock of flamingos in her Transfiguration O.W.L. exam.

Appearances
Harry Potter and the Philosopher's Stone

Harry Potter and the Chamber of Secrets

Harry Potter and the Goblet of Fire

Harry Potter and the Order of the Phoenix