We do not currently know of any antonyms for Pegasus.
The proper noun Pegasus is defined as:
A winged horse fabled to have sprung from the blood of Medusa when she was slain. He is noted for causing, with a blow of his hoof, Hippocrene, the inspiring fountain of the Muses, to spring from Mount Helicon. Bellerophon tamed and rode upon Pegasus when he defeated the Chimaera.