Animagus

An Animagus is a witch or wizard able to transform at will into a specific animal, a term Harry Potter and the Prisoner of Azkaban formally introduces in Transfiguration class, though readers have seen the skill since the first chapter of the series, when Professor McGonagall unbends from a tabby cat on Privet Drive. The transformation differs from ordinary Transfiguration: it is self-applied, repeatable at will, and fixed to a single animal form that is not chosen by its owner. Achieving it takes years of disciplined study, and a botched attempt can go very badly wrong. Because the ability lends itself to concealment, wizarding law requires every Animagus to register with the Ministry of Magic, recording their animal form and its markings.