Animagus

In the Potterverse, an Animagus is a witch or wizard with the ability to transform into an animal.

The process of becoming an Animagus is difficult, and attempts are carefully monitored by the Ministry of Magic. However, in their fifth year at Hogwarts, James Potter, Sirius Black and Peter Pettigrew achieved this complex bit of magic. Other known Animagi include Minerva McGonagall, Rita Skeeter, and Agent Paddlebrains.

Being an Animagus is a common Sue trait. If a Sue can turn into a magical creature like a dragon or a phoenix, it is a charge.