One of the most iconic characters in ancient Greek mythology.

The beautiful mortal virgin Medusa, whose name means guardian or protectress, caught the eye of Poseidon, who had his way with her in a temple of Athena, who then took revenge upon the young woman by transforming her beautiful hair into hissing venomous snakes. And she was so hideously ugly that anyone who looked at her would be petrified. Literally turned into stone.

A mortal Gorgon, unlike her two immortal Gorgon sisters, she was vulnerable. The hero Perseus managed to kill her by looking at her reflection in a mirrored shield which he had been given by Athena. Perseus then gave Medusa’s head to Athena, who placed it in the centre of her aegis (her divine breastplate) where, known as the Gorgoneion, it became an apotropaic protective symbol—it was believed to ward off evil and enemies. On Athena’s aegis and shield, it represents divine power, protection, and fearlessness.

Medusa was pregnant by Poseidon when Perseus beheaded her... so at the point of her death, Pegasus the winged horse, and Chrysaor (a giant wielding a golden sword) sprang from her body.

(aside: Natalie Haynes' 'Divine Might' is wonderful)