Metamorphoses Book 12: Caenis
Caenis was a beautiful girl who would marry no one. Neptune raped her and then offered to grant her one wish. She answered: "'This wrong you've done me needs an enormous wish -- / Put pain like that beyond my power. Grant me / To cease to be a woman -- everything / That gift will be to me.'" Book 12 -- Caenis, line 201-4 Neptune granted the wish and then she became Caeneus. He also made her invulnerable to spears and arrows.