The story of Cancer:

The sign of Cancer stems from one of the 12 Trials of Heracles. While battling the Hydra, Hera sent down a giant crab to thwart Heracles efforts. The crab was only a nuisance at most as Heracles simply crushed the crab under his foot just before he defeated the Hydra. Hera honored the crab's attempt at stopping her most hated of Zeus' children by placing it amongst the stars.