Which Greek monster had snakes for hair?

The Greek monster traditionally depicted with snakes for hair was Medusa.

Medusa was one of the three Gorgons in Greek mythology and the only one who was mortal. She was generally portrayed with living snakes in place of hair, and her gaze was said to turn anyone who looked directly at her to stone. Her sisters, Stheno and Euryale, were also Gorgons, but ancient stories normally distinguish Medusa as the one Perseus could kill.

Perseus was sent to obtain Medusa’s head and received divine assistance, including a reflective shield from Athena. By looking at Medusa’s reflection rather than meeting her eyes, he approached safely and beheaded her. From her blood sprang Pegasus and Chrysaor, according to myth. Perseus later gave the head to Athena, who placed it on her protective armor or shield.

The familiar story that Medusa was originally beautiful and was transformed after an encounter in Athena’s temple comes mainly from the Roman poet Ovid. Earlier Greek traditions and artworks often presented her as a monstrous figure from the start. In Classical art, her image—the gorgoneion—was also used as a protective symbol to ward off evil.