Animagus
In the Harry Potter universe, an animagus is a wizard or witch capable of changing into the form of an animal. Several animagi have been mentioned in the Harry Potter books, most famously Professor McGonagall, who is depicted in the first chapter of Harry Potter and the Philosopher's Stone as being able to change into a cat.
Becoming an animagus is difficult, taking years, and a wizard/witch is able to turn into a specific form - it is not a general talent allowing one to become any animal. The choice of animal is also not theirs to make -- it instead reflects something of their inner nature.
Spoiler warning: Plot or ending details follow.
Officially, all animagi have 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. However, several unauthorised animagi have been mentioned in the series.
Here follows a listing of all known animagi.
Registered animagi
The only registered animagus we are told about is Minerva McGonagall, who can turn into a 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.
