An Animagus (portmanteau of animal and magus) is a witch or wizard who can turn into a particular animal or magical creature at will. This ability is not innate: it must be acquired by magical means. All Animagi must register at a central authority by law, though a number of characters are revealed over the course of the series to have remained unregistered illegally: James Potter, Sirius Black, Peter Pettigrew and Rita Skeeter. Minerva McGonagall was a ministry registered animagus taking a form of tabby cat.