(Greek; Pegasus, Latin). The inspiration of poetry, or, according to Boiardo (Orlando Inamorato), the horse of the Muses. A poet speaks of his Pegasus, as “My Pegasus will not go this morning,” meaning his brain will not work. “I am mounting Pegasus” —i.e. going to write poetry. “I am on my Pegasus,” i.e. engaged in writing verses.
Pegasus or Pegasos, according to classic mythology, was the winged horse on which Bellerophon rode against the Chimaera. When the Muses contended with the daughters of Pieros, Helicon rose heavenward with delight; but Pegasos gave it a kick, stopped its ascent, and brought out of the mountain the soul-inspiring waters of Hippocrene [Hip'-po-creen].
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894