Pegasus (the winged horse) is a northern constellation. It is one of the 88 modern constellations, and was also one of the 48 constellations listed by Ptolemy. Its three brightest stars together with Alpha Andromedae form the large asterism known as the Square of Pegasus.
In Greek Mythology, a winged horse that was the child of Poseidon and the gorgon Medusa. 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.
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, 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.
Pegasus was eventually turned into a constellation (see above), but a single feather fell to the earth near the city of Tarsus (hence its name).