In Greek mythology, Medusa (Greek: ???????), was a monstrous female character whose gaze could turn people to stone.

Some classical references describe her as one of three Gorgon sisters. Medusa, Stheno, and Euryale were monsters with brass hands, sharp fangs and hair of living, venomous snakes. The Gorgons and their other sisters the Graiae (and possibly the Hesperides) were daughters of Phorcys and Ceto. Some versions read that Medusa's sisters transformed themselves into monsters in order to protect Medusa.

In the most well known version of the myth, Medusa originally started out as a beautiful woman. She had sex with -- or was "raped" by -- Poseidon in Athena's temple. Poseidon was an arch-rival of Athena's since at one time he vied for patronage of Athens; the soon-to-become Athenians chose Athena's offering of the olive tree over Poseidon's offering of horses or a spring of water.
Perseus beheads Medusa
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Perseus beheads Medusa

Upon discovery of the desecration of her temple, Athena changed Medusa's form to match that of her sister Gorgons as punishment. Medusa's hair turned into snakes and her glance would turn all living creatures to stone. She was banished beyond the Hyperborean lands.

While Medusa was pregnant by Poseidon, she was beheaded in her sleep by the hero Perseus with help from Athena and Hermes, who supplied him with winged sandals, Hades' cap of invisibility, a sickle, and a mirrored shield. In some versions of the legend, it was upon seeing her own reflection in the mirror that Medusa was turned to stone. According to other versions, Perseus was either blindfolded or was able to slay Medusa by looking at her reflection in the mirror instead of directly at her to prevent being turned into stone. In any case, Perseus severed Medusa's head and from her neck came her offspring: Pegasus and Chrysaor. Perseus used Medusa's head to rescue Andromeda, kill Polydectes, and, in some versions, petrify the Titan Atlas. Then he gave it to Athena, who placed it on her shield Aegis.

Another well known version is that the once human Medusa claimed to be more beautiful than the Greek Goddess of love and beauty, Aphrodite. This caused her to turn Medusa and her sisters into Gorgons.

Wiki: http://en.wikipedia.org/wiki/Medusa_(mythology )

Medusa is a fictional character in the Marvel Comics' universe. Her real name is Medusalith Amaquelin. She belongs to the race of Inhumans, a species of prehistoric earthlings mutated by the Terrigen mists produced deep under the Inhuman city-state of Attilan, presently located in the oxygen-rich Blue Area of the moon. Medusa is married to Black Bolt and they have a son named Ahura. Crystal, the wife of the Avengers Quicksilver, is Medusa's sister.

Medusa first appeared in Fantastic Four #36, making her one of the oldest characters in the Marvel Comics' universe. At that time, she suffered from amnesia and affiliated herself with the Frightful Four, Silver Age nemeses of the Fantastic Four. After regaining her senses, Medusa returned to Attilan and later still she joined the Fantastic Four as a temporary member filling in for Susan Richards, aka Invisible Girl, who was on leave rearing her son Franklin Richards.

Medusa appeared in the Ultimate Marvel Universe during the Ultimate Fantastic Four issue, Annual 1: Inhuman. This version had a head full of snakes like the mythical creature she is named after. She was still Black Bolt's wife and appeared to share the same relationship to the other Inhumans with her regular counter-part.

Medusa possesses a head of thick red hair that's six feet long. Her mutation gives her psychokinetic control over every strand, each of which possesses greater tensile strength, modus of elasticity and shear resistance than an iron wire of the same thickness. In combat, Medusa uses her hair to wield weapons, bind her foes, etc.

http://en.wikipedia.org/wiki/Medusa_%28comics%29