"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)
Animagus Transfiguration is the magical process of taking on the form of an animal, or afterward, reverting back again to human form. The animal form is sometimes referred to as the witch or wizard’s Animagus form.
See the Animagus entry for further details about wizarding folk who can perform this highly complex and dangerous magic.
"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.
From the Web
Writing by J K Rowling on WizardingWorld (Pottermore): Uagadou
Tumbler (JonSnowWhite): Animagi by J K Rowling
The Animal Paradox as represented in the Harry Potter series by booksblnktsandt
Harry Potter Wiki: Animagus
MuggleNet: Animagus Forms of the New Generation
The-Leaky-Cauldron: Animal Attraction by Clarissa P
DwellingOnDreams blog and podcast: Animagus Forms, and The People Who Have Them
WizardingWorld (Pottermore) features: