This is an advanced magic that allows a witch or wizard to transform into an animal at will.
Animagi are rare and are required to be registered with the Ministry of Magic. There have only been seven in the last century that have been registered, either due to the complexity of the spell or the fact that there are unpleasant side-effects is the spell should go wrong. The witch or wizard in question cannot choose the animal which it transforms into, just like how they cannot choose which animal their Patronus spell is.
The only known registered Animagus is Minerva McGonagall, who can transform into a cat. There are a number of illegal Animagi, including James Potter, a stag, Sirius Black, a dog, Peter Pettigrew, a rat, and Rita Skeeter, a beetle.
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