Callisto

Callisto was an Arcadian nymph, the daughter of Lycaon and a follower and favorite of Artemis. She was raped by Zeus, who disguised himself as Artemis in order to get close to her. Callisto tried to hide her loss of virginity, but after she became pregnant, Artemis exiled her. When Callisto gave birth to a son, Arcas, Hera was enraged and turned her into a fearsome bear.

As a bear, Callisto roamed Arcadia for fifteen years, until one day she encountered her son out hunting. She tried to approach the boy, but he was frightened and attempted to kill her. In order to prevent the matricide, Zeus turned both mother and son into constellations, and set them in the sky as Ursa Major and Minor (Great Bear and Little Bear). Hera was further angered by this honor given to her rival and persuaded Tethys and Oceanus to refuse to allow the mother and son to bathe in their waters--hence the two constellations never descend below the horizon into the sea.