In Greek mythology, Pegasus, or Pegasos was a winged horse that was the child of Poseidon and the Gorgon Medusa, despite both of them being human in shape.

Descriptions vary as to its birth, some say that it sprung from Medusa's neck as Perseus beheaded her, which fits in with his parentage; others say that he was born of the earth as Medusa's blood spilled onto it, in which case Poseidon would not be his father. The word Pegasus is derived from the Greek for the word spring. Pegusus is often used to refer to winged horses in general, but to be correct they should called pterippi.

Pegasus aided the hero Bellerophon in his fight against both the Chimera and the Amazons. There are varying tales as to how Bellerophon found Pegasus, some say that the hero found him drinking at the Pirenean spring and that Polyidus told Bellerophon how to find and tame him, others that either Athene or Poseidon brought him to Bellerophon.

Prior to aiding Bellerophon, Pegasus brought thunderbolts to Zeus, and following Bellerophon's death he returned to Mount Olympus to aid the gods. Two springs were supposedly created when Pegasus's hoof struck the earth; one on Mount Helicon at the behest of Poseidon to prevent the mountain swelling too much and another at Troezen.

In his later life, Pegasus took a wife, Euippe (or Ocyrrhoe), by whom he had two children, Celeris and Melanippe. This family is the origin of the pterippi. Pegasus was eventually turned into a constellation (See: Pegasus (constellation)), but a single feather fell to the earth near the city of Tarsus (hence its name).