Here's another version of the myth.
Medusa was described as the most beautiful woman in Athens.
Her sisters were Stheno and Euryale, but she was the most beautiful of the three. They were the daughters of the gods Phorcys and Ceto though, somehow, only Medusa was human, while the other two were immortal. They were all wise and were priestessess of Athena (Medus'a name comes from metis, meaning wisdom).
One day, because of Medusa's beauty, Poseidon saw her and became mad with lust; he went to Athena's temple and raped her. All three sisters became angry and when Athena found out she was as well. Because she was a virgin goddess it was a great sin for Medusa to have been raped and even moreso to have been raped in Athena's own temple. In a rage, Athena turned the sisters into monsters, but because it was Medusa's beauty that had gotten her raped, she made her so hideous that any man who looked upon her would be turned to stone. The sisters became so vicious toward men and took great pleasure in killing them until Persues dispatched the three.
After Medusa lost her head Pegasus leapt from her body (though some interpritations say it was Chrysaor, another winged horse-black I think) and so was born and taken to be raised by the Muses.
