Reason For Being A Constellation
Karkinos is a giant crab. During Herakles second labor (slaying the Hydra) Hera sent the crab in attempt to distract Heracles and thus give the Hydra the upper hand. Karkinos however failed and one swift kick from Heracles cracked the shell and killed Karkinos.
As A Constellation
As a reward for giving its life doing Hera's bidding, Hera placed Karkinos among the skies as the constellation/zodiac symbol Cancer. Cancer is near the constellation Hydra.
Gallery
Cancer's astrological symbol
The Constellation of Cancer