Animagus

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 the transformation can go
horribly wrong. Animagi can assume only one form. Furthermore, 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.

For historical information, see magus.

Spoiler warning: Plot and/or
ending details follow.

It took Peter Pettigrew, James Potter, and Sirius Black (James
and Sirius are said to be the cleverest students in their school at
the time) the best part of three years (between their second and
fifth school years) to work out how to do it; it is not clear how
long the process would take a fully-trained wizard or witch to
accomplish.

Whilst in animal form, the witch or wizard retains all sensory
perception and cognition.
Simply put, they have the abilities of the animal they have
transformed into, while retaining the mentality of a human. 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.

Another wizard or witch can use his/her wand to force an
Animagus to transform back to human form, producing a flash of
blue-white light in the process (Prisoner of
Azkaban).

Registered Animagi

In the third book, which takes place during 1993 and 1994, it is mentioned that only seven
Animagi have so far registered during the entire 20th
century. The only known one is Minerva McGonagall, who transforms into a
cat. Professor Lupin is a werewolf and not an Animagus.

Unregistered Animagi

As revealed in Harry Potter and
the Prisoner of Azkaban, James Potter was a stag-animagus,
Sirius Black was a dog-animagus, and
Peter Pettigrew a rat-animagus; all three
of them were/are unregistered, having managed to master the
animagus transformation during their fifth year at Hogwarts.

Ron's pet rat, was really Peter Pettigrew in disguise, for
twelve years. He is the only animagus so far in the series to stay
in animal form for such a long period of time.

In Harry Potter and the Goblet
of Fire, it is revealed that Rita
Skeeter, a pesky tabloid journalist, obtains all of her secrets
in the form of a beetle.

Aside from Minerva McGonagall, it is unknown who the other six
registered Animagi of the 20th
Century are.

Historical Animagi

In both the Harry Potter computer
games by Electronic Arts and the Harry Potter
trading card game, there is 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.