"Your father and Sirius here were the cleverest students in the school, and lucky they were, because the Animagus transformation can go horribly wrong -- one reason the Ministry keeps a close watch on those attempting to do it. Peter needed all the help he could get from James and Sirius. Finally, in our fifth year, they managed it. They could each turn into a different animal at will."
-- Remus Lupin (PA18)
The Transfiguration by which an Animagus takes his or her animal form, or reverts to human form from animal form. The animal form is sometimes referred to as the witch or wizard’s Animagus form.
See Animagi entry for further details about wizarding folk who can perform this highly complex and dangerous magic.
References from the canon
- Part of the process of becoming an Animagus requires you to carry a leaf from a Mandrake in your mouth for an entire month.(Bloomsbury New Editions, 2015)
- McGonagall transforms into a cat (PS1, PA6).
- Sirius Black, James Potter, and Peter Pettigrew became Animagi while students at Hogwarts (PA17, PA18, PA19).
- Animagi are supposed to be registered with the Ministry of Magic (PA18) but there are unregistered ones around, for example Rita Skeeter(GF37).
- In The Tales of Beedle the Bard, Babbity Rabbity transforms into a white rabbit
"animal" L. animal + "magus" Pers. magic user
For an excellent discussion of the legend and mythology of animal transformations, see pages 9-15 of David Colbert's Magical Worlds of Harry Potter.