Medusa was a terrible monster who had laid waste to the country. She was once a beautiful maiden whose hair was her chief glory, but as she dared to vie in beauty with Athena, the goddess deprived her of her charms and changed her beautiful ringlets into hissing serpents. She became a cruel monster of so frightening an aspect that no living thing could behold her without being turned into stone. All around the cavern where she dwelt might be seen the stony figures of men and animals which had chanced to catch a glimpse of her and had been petrified with the sight. Perseus, favored by Athena and Hermes, the former of whom lent him her shield and the latter his winged shoes, approached Medusa while she slept, and taking care not to look directly at her, but guided by her image reflected in the bright shield which he bore, he cut off her head and gave it to Athena, who fixed it in the middle of her Aegis.
from Bulfinch's Mythology
Many later images of Medusa thwart our expectations of what she should look like. From the previous story, we are led to believe that she was a creature so hideous in appearance that her very glance could petrify the viewer. However, in works such as the Medusa Rondanini, Medusa has the face of a beautiful woman. It is only her expression of deep sorrow - and the intertwined snakes around her head - that hint that this is a representation of the monster of myth and legend. This manner of depiction reveals that Medusa was originally a lovely woman - it was her tragedy that she was foolish enough to compare herself to a goddess.