Harry’s father James Potter spent three years learning to become an Animagus, a wizard who can transform into an animal. He ended up perfecting it and became a stag Animagus. And, according to Reddit user alexi_lupin, nobody tells Harry that his dad could turn into a stag in the movies, which “cheapens the Patronus, cheapens Harry's relationship with his father (which never recovers), and also Snape's reveal later.”