They were three powerful, winged daemons named Medusa, Sthenno and Euryale. Of the three sisters only Medusa was mortal.
King Polydectes of Seriphus once commanded the hero Perseus to fetch her head. He accomplished this with the help of the gods, who equipped him with a reflective shield, a curved sword, winged boots and helm of invisibility. When he fell upon Medusa and decapitated her, two creatures sprang forth from the wound; the winged horse Pegasus and the giant Chrysaor. Perseus fled with the monster’s head in a sack with her two angry sisters chasing close on his heels.