The Greek Myth of Cancer goes like this: The Hydra of Lerna was a colossal serpent that frightened the region of Lerna, in Peloponnese, destroying herds and crops. The Hydra had nine heads, and to each one cut off, two were born in the place. The second of the twelve labors imposed on Hercules by King Eurystheus was to rid the region of this terrible monster.
Hercules planned to get rid of the Hydra by beheading their heads. To complete his work, he enlisted the help of his friend Iolaus. To prevent the continual resurgence, Hercules cut them off and Iolaus cauterized the place preventing the appearance of new heads. After eliminating all the mortals, while preparing to bury the last one, Hera, who hated Hercules for being the son of an adulterous relationship between Zeus and a mortal, sent a huge crab to stop him.
The hero crushed it with his feet and managed to complete his work. Iolaus set fire to the Hydra stronghold, burning its remains preventing it from resurrecting. Hera collected the crab and raised it to the sky in the shape of a constellation. In Latin, cancer means crab.
Didn't you find the Greek Myth of the sign of Cancer interesting?