Nonnus, Dionysiaca 31. 13 ff (trans. Rouse) (Greek epic C5th A.D.) : “As Medousa was slain [by Perseus], the neck was delivered of its twin...

From Medusa's decapitated body the giant Chrysaor and the winged horse Pegasus, her son by Poseidon, sprang forth.  Edward Burne-...