Pegasus

Pegasus springs from the neck of Medusa when Perseus beheads her — a child of Poseidon, which fits, since Poseidon is god of horses as well as the sea. His brother, born at the same moment, is the warrior Chrysaor.

He could not be caught by ordinary means. Bellerophon slept in Athena's temple and woke with a golden bridle beside him; with it he took Pegasus while the horse drank at the spring of Peirene, and the two of them killed the Chimera from the air.

The partnership ends badly, and the fault is the rider's. Bellerophon decided that having done the impossible he belonged on Olympus, and flew Pegasus up toward it. Zeus sent a gadfly, the horse threw him, and he fell — lamed and blinded in most accounts, wandering the earth alone, avoided by gods and men. Pegasus flew on without him.

Zeus kept the horse. Pegasus was given a place in the divine stables carrying thunderbolts, and finally set among the stars as a constellation. Where his hoof struck Mount Helicon a spring broke out, the Hippocrene, sacred to the Muses — which is how a horse became a symbol for poetic inspiration.

The Hippocrene spring gave the horse a second career. Because the Muses' fountain opened under his hoof, Pegasus became the emblem of poetry itself, and 'to mount Pegasus' meant to write verse. He is one of very few figures in Greek myth whose meaning changed completely — from a monster's orphan to the sign over a poet's door.