The Medusa was the daughter of Phorkys and Keto, the children of Gaia (Earth) and Okeanos (Ocean). She was one of the three sisters known as the Gorgons. The other two sisters were Sthenno and Euryale. Medusa was the only mortal out of the three. She was once very beautiful and lived far in the north were the sun didn't visit. Being very curious, she wanted to see the sun, and asked the Goddess Athena for permission to visit the south. Athena refused to allow her to visit. The medusa got angry and dared to say that Athena hadn't given her permission because she was jealous of her beauty. that was it! Athena was angered and punished her by turning her hair into snakes and cursing her by making her so ugly that who ever lookes at her eyes would turn into stone.
Yes she was very beautiful there are varying stories about how she got turned into the monster she became. One version says she bragged about her beauty and Athena turned her into the monster to teach her a lesson. Another says she begged Athena to let her travel south to see the sun, and Athena refused, when Medusa heard this she got angry and accused Athena of being jealous of her beauty... Yet another says she was seduced by one of the gods/lovers of Athena and when Athena found out she turned into a hideous creature, and every man would try to kill her and the sight of her would turn men to stone.