One version of the myth is that Kallisto was a Hunter of Artemis. Zeus fell in love with Kallisto and in order to get close to her, he disguised himself so as to look like Artemis, and while she was separated from the group, seduced her in this form. Kallisto would become pregnant during this encounter, and tried to keep it from showing. Eventually, Artemis and the other Hunters discovered her condition while bathing and the goddess was furious. She told Kallisto to run, for she and the other Hunters would hunt her down if they came across her again. Kallisto (as a bear) eventually gave birth to a human son by the name of Arkas, but she was later tragically killed by hunters (though it remains unclear whether Kallisto's killers were the Hunters of Artemis). Out of pity, Zeus transformed his former lover into a constellation; the constellation which would become known as Ursa Major. 
Zoë uses Ursa Major to lead everyone west through the Junkyard of the Gods. When Percy Jackson referred to it as the bear, Zoë called it a "worthy opponent," suggesting she had fought it at some point in her past.
- If Zoë Nightshade was indeed the one who hunted the bear that became Ursa Major, that would mean Kalliso was in fact killed by a Hunter of Artemis.