Pegasus was the winged horse on which Bellerophon rode when slaying the Chimaera. It was said he could not be ridden without a special bridle, which Bellerophon acquired with the help of Athena. Before being tamed, Pegasus lived on Mount Helicon with the Muses.
Pegasus was the son of Medusa and Poseidon, springing fully-grown from his mother's neck after she was beheaded by Perseus. Pegasus had been conceived before Medusa was changed into a Gorgon.
When Bellerophon tried to fly to Mount Olympus, Zeus caused Pegasus to rear back, throwing off its rider back down to Earth. Zeus then used Pegasus to carry Zeus' thunderbolts, for which Pegasus was rewarded with a constellation.