Harry Potter’s Patronus in J.K. Rowling’s novels is a stag. The silver animal appears when Harry casts the Patronus Charm, a protective spell used against Dementors and other dark forces.
The stag has personal meaning in the story because Harry’s father, James Potter, could transform into a stag as an Animagus. Harry’s Patronus therefore reflects a family connection as well as his own courage and protective instincts. The form becomes especially important during the events around the lake in Harry Potter and the Prisoner of Azkaban.
A Patronus is not necessarily the same thing as an Animagus transformation. An Animagus is a witch or wizard who can deliberately change physical form, while a Patronus is a magical guardian produced by a spell. James’s Animagus form and Harry’s Patronus happen to match, which is why the distinction is often discussed by readers.