The myth of Cancer originates in one of the Twelve Labors of Hercules, most specifically the one where he had to defeat the Lernean Hydra. Juno sent Karkinos (a crab) to distract Hercules during the battle, and here the myth has two different ends. In one, Hercules kicks the crab with so much strength that he sends it to the sky where it transforms into a constellation. In the other one, Hercules simply smashes the crab with his foot, and Juno, seeing how heroic was its effort, gave it a place in the sky. It is theorized by some scholars that the addition of Karkinos in the myth was just to make the 12 labors coincide with the 12 signs of the zodiac.
