What animal is Pegasus?

Pegasus is a horse with wings

His father was Poseidon the Greek sea god, when he was being a horse, and his mother was Medusa, one of the three Gorgon sisters. Pegasus had a twin brother, Chrysaor, who wasn't a horse but was a flying pig (boar). Legend says the twins were born (you don't want to know how) when Perseus, son of Zeus and Danaë, beheaded Medusa.