Pegasus was a beautiful winged horse who sprang from the body of Medusa when she was slain by the hero Perseus, the son of Zeus and Danaë. Spreading out his wings he immediately flew to the top of Mount Olympus, where he was received with delight and admiration by all the immortals.
A place in his palace was assigned to him by Zeus, who employed him to carry his thunder and lightning. Pegasus permitted none but the gods to mount him, except in the case of Bellerophon, whom, at the command of Athene, he carried aloft, in order that he might slay the Chimæra with his arrows.
The later poets represent Pegasus as being at the service of the Muses, and for this reason he is more celebrated in modern times than in antiquity. He would appear to represent that poetical inspiration, which tends to develop man's higher nature, and causes the mind to soar heavenwards.
The only mention by the ancients of Pegasus in connection with the Muses, is the story of his having produced with his hoofs, the famous fountain Hippocrene. It is said that during their contest with the Pierides, the Muses played and sang on the summit of Mount Helicon with such extraordinary power and sweetness, that heaven and earth stood still to listen, whilst the mountain raised itself in joyous ecstasy towards the abode of the celestial gods. Poseidon, seeing his special function thus interfered with, sent Pegasus to check the boldness of the mountain, in daring to move without his permission.
When Pegasus reached the summit, he stamped the ground with his hoofs, and out gushed the waters of Hippocrene, afterwards so renowned as the sacred fount, whence the Muses quaffed their richest draughts of inspiration.
From: Berens, E.M. The Myths and Legends of Ancient Greece and Rome. New York: Maynard, Merril, & Co., 1880. Text in the public domain.