Lewis & Short

1. Pēgăsus (-os), i, m., = Πήγασος, the winged horse of the Muses, who sprang from the blood of Medusa when she was slain, and with a blow of his hoof caused the fountain of the Muses (Hippocrene) to spring from Mount Helicon. Bellerophon afterwards caught him at the fountain of Pirene, near Corinth, and, with the aid of his hoofs, destroyed the Chimœra. But when Bellerophon wished to fly on the back of Pegasus to heaven, the latter threw him off and ascended to the skies alone, where he was changed into a constellation, Ov. M. 4, 785; 5, 262 sq.; id. F. 3, 458: ales, Hor. C. 4, 11, 27; Hyg. Fab. 151; id. Astr. 2, 18.
Applied in jest to a swift messenger, Cic. Quint. 25, 80.
Of winged horses in gen., Plin. 8, 21, 30, § 72; cf.: sunt mirae aves cornutae (in Africā) et equinis auribus Pegasi, Mel. 3, 9.
Hence,

    1. 1. Pēgăsēïus, a, um, adj., Pegasean, i. e. poetic: melos, Pers. praef. 14.
    2. 2. Pēgăsĕus (Pēgăsēus, Mart. Cap. 9 fin.), a, um, adj., of or belonging to Pegasus, Pegasean: volatus, Cat. 55, 24: habenae, Claud. in Ruf. 3, 262: aquae, Hippocrene, id. Epigr. 5, 4.
      Pegaseum stagnum, a lake in lonia, Plin. 5, 27, 31, § 115: aetas Pegaseo corripiet gradu, i. e. with rapid step, Sen. Troad. 385.
    3. 3. Pēgă-sis, ĭdis, f. adj., of Pegasus: Pegasides undae, the waters of Hippocrene, the fountain of the Muses, Ov. Tr. 3, 7, 15: unda, Mart. 9, 59, 6.
      Subst.: Pēgăsĭdes, the Muses, Ov. H. 15, 27; Prop. 3 (4), 1, 19.
      Pēgă-sis, ĭdis, f., = Πήγή, a fountain-nymph: Pegasis Oenone Phrygiis celeberrima silvis, Ov. H. 5, 3.