According to Greek mythology, Pegasus was born when the hero Perseus slew the Gorgon Medusa. Medusa's gaze was so deadly it turned anyone who looked at her to stone, and instead of hair, her head was covered with snakes. When Perseus cut off her head some of her blood fell into the foam of the sea, and it was from this gruesome but highly magical combination that the flying horse was formed.
He immediately flew off to Mount Helicon, where the nine Muses lived, and as he landed on the earth again his hooves opened a spring of clear water. This was called The Horse's Fountain, and gave poetic inspiration to anyone who drank from it. Because of this, the Flying Horse became a symbol of poetry and the creative arts.
The only person to ride Pegasus was the hero Bellerophon. The goddess Athene gave him a golden bridle, and with this he was able to capture the horse as he was drinking. Bellerophon then rode Pegasus to fight the Chimaera - monster with the head of a lion, body of a goat and tail of a snake. Because of his courage in killing the Chimaera, the gods officially gave Pegasus to Bellerophon.
Bellerophon had many adventures riding Pegasus, and became famous and proserous while he had the favour of the gods. But, unwisely, he attempted to fly to the home of the gods on Mount Olympus. He should have known that the gods would never allow this. Zeus sent a gadfly to sting Pegasus, making him throw Bellerophon off - some say he fell to his death, others say he was seriously injured and never rode or fought again.
Zeus then claimed the horse, and used him to carry his thunderbolts for a while, but the flying horse was never destined to be a beast of burden, even for the king of the gods.
Eventually Zeus set him free, and he flew to the night sky to become a constellation among the stars.
Pegasus is portrayed differently in the Clash of the Titans movies. In the movies, Pegasus is ridden by Perseus instead of Bellerophon and his coat is black instead of white. If you haven't seen them, most Comcast offers include a variety of movie channels where you should be able to check them out.