In the Harry Potter books, an animagus is a wizard or witch capable of turning into an animal and back at will.
Becoming an animagus is difficult and takes years. Animagi can assume only one form. Furthermore, it seems that the choice of animal is not up to the user, but instead reflects something of one's inner nature. The series features several animagi, most famously Professor McGonagall who spends much of the first chapter of Harry Potter and the Philosopher's Stone as a cat.
All animagi are required to register themselves with the Ministry of Magic, as transforming into an animal can be extremely dangerous; also, registering their specific markings allows the Ministry to track crimes committed in animal form, or wizards/witches who might use an animal form to escape justice. The process does not appear to be very effective, likely due to the difficulty of enforcing it.
Registered animagi
In the first book, which takes place during 1991 and 1992, it's mentioned that only four animagi have so far registered during the entire 20th century. The only known one is Minerva McGonagall, cat. Professor Lupin is a werewolf and not an animagus.
Unregistered animagi
Historical animagi
In both the Harry Potter computer games by Electronic Arts and the Harry Potter trading card game, there's mention of certain historical animagi who presumably lived before any kind of registration process. These people are Falco Aesalon (an Ancient Greek who could turn into a falcon, and is the first recorded example of an Animagus) and the witches Morgana Le Fay and Cliodne who lived in medieval times and were both bird animagi.
