What do the ancient texts say ?

In the Metamorphoses (IV, 796), Ovid tells the story of Medusa from the time when she was not yet a monster:
"She was once most beautiful, and the jealous aspiration of many suitors. Of all her beauties, none was more admired than her hair (...) They say that Neptune, lord of the seas, violated her in the temple of Minerva. Jupiter’s daughter [Minerva] turned away, and hid her chaste eyes behind her aegis. So that it [the desecration of her temple] might not go unpunished, she changed the Gorgon’s hair to foul snakes. And now, to terrify her enemies, numbing them with fear, the goddess wears the snakes, that she created, as a breastplate."
Pausanias, a Greek geographer and historian of the 2nd century AD, describes the famous cult statue of Athena Parthenos:
"The statue of Athena is upright, with a tunic reaching to the feet, and on her breast the head of Medusa is worked in ivory. She holds a statue of Victory about four cubits high, and in the other hand a spear; at her feet lies a shield and near the spear is a serpent."
(Description of Greece, I, 24, 7 ; translation WHS Jones)