How did the constellation Pegasus get its name?

The constellation Pegasus was named after the winged horse of Greek mythology. He sprang from the blood of the Gorgon Medusa after she was slain by Perseus. Zeus, the chief god, placed the horse in the night sky among other demigods and heroes.

One of Pegasus' masters was Bellerophon, who attempted to ride the horse to Mount Olympus, the home of the gods. Offended, Zeus sent a gadfly to sting Pegasus. Pegasus then reared and threw Bellerophon off, but continued alone to Mt. Olympus, thereby earning himself a place among the stars.

Pegasus is a constellation that can be seen in the northern latitudes during the autumn. It's one of 48 constellations catalogued by Ptolemy.