Pegasus

Pegasus (from Gr. irn-y6s, compact, strong), the famous winged horse of Greek fable, said to have sprung from the trunk of the Gorgon Medusa when her head was cut off by Perseus. Bellerophon caught him as he drank of the spring Peirene on the Acrocorinthus at Corinth, or received him tamed and bridled at the hands of Athena (Pindar, 01. xiii. 63; Pausanias ii. 4). Mounted on Pegasus, Bellerophon slew the Chimaera and overcame the Solymi and the Amazons, but when he tried to fly to heaven on the horse's back he threw him and continued his heavenward course (Apollodorus ii. 3). Arrived in heaven, Pegasus served Zeus, fetching for him his thunder and lightning (Hesiod, Theog. 281). Hence some have thought that Pegasus is a symbol of the thundercloud. According to 0. Gruppe (Griechische Mythologie, i. 75, 123) Pegasus, like Anion the fabled offspring of Demeter and Poseidon, was a curse-horse, symbolical of the rapidity with which curses were fulfilled. In later legend he is the horse of Eos, the morning. The erroneous derivation from irnyrl, "a spring of water," may have given birth to the legends which connect Pegasus with water; e.g.  that his father was Poseidon, that he was born at the springs of Ocean, and that he had the power of making springs rise from the ground by a blow of his hoof. When Mt Helicon, enchanted by the song of the Muses, began to rise to heaven, Pegasus stopped its ascent by stamping on the ground (Antoninus Liberalis 9), and where he struck the earth Hippocrene (horsespring), the fountain of the Muses, gushed forth (Pausanias ii. 31, ix. 3r). But there are facts that speak for an independent mythological connexion between horses and water, e.g. the sacredness of the horse to Poseidon, the epithets Hippios and Equester applied to Poseidon and Neptune, the Greek fable of the origin of the first horse (produced by Poseidon striking the ground with his trident), and the custom in Argolis of sacrificing horses to Poseidon by drowning them in a well. From his connexion with Hippocrene Pegasus has come to be regarded as the horse of the Muses and hence as a symbol of poetry. But this is a modern attribute of Pegasus, not known to the ancients, and dating only from the Orlando innamorato  of Boiardo.

See monograph by F. Hannig, Breslauer philologische Abhandlungen (1902), vol. viii., pt. 4.