MagicPhenomenon
Animagus
An Animagus is a witch or wizard who can turn into a particular animal at will. The form is registered with the Ministry, except when the transformation was learned in secret. Minerva McGonagall’s cat form is legal. James, Sirius, and Peter became a stag, a dog, and a rat without registering, so they could keep Remus Lupin company at the full moon. Rita Skeeter’s beetle form is another unregistered case.
In the books
Harry Potter and the Prisoner of AzkabanChapter: Moony, Wormtail, Padfoot and Prongs
The chapter in the Shrieking Shack lays out the four names and the animal forms. McGonagall’s tabby is on the page from the first chapter of the series, before the word Animagus is explained.