Star Tales - Cancer (ianridpath.com)
The crab is a minor character in one of the labours of Heracles (the Greek name for Hercules). While Heracles was fighting the multi-headed monster called the Hydra in the swamp near Lerna, the crab emerged from the swamp and added its own attack by biting Heracles on the foot. Heracles angrily stamped on the crab, crushing it. For this modest contribution to history, we are told that the goddess Hera, the enemy of Heracles, put the crab among the stars of the zodiac. Its name in Greek was Καρκίνος (Karkinos), or Carcinus in Latin transliteration.