Carcinus Picture

An ally of the Lernaean Hydra, Carcinus rushed to the she-serpent's aid when the hero Hercules attacked her. Despite their efforts both monsters were killed, though Hera immortalized the pair by placing them in the stars as the constellations Hydra and Cancer.

Carcinus' history is unknown -- we aren't told who he's related to (he may very well just be a normal crab), if he was acquainted with Hera like most monsters that fought Heracles, or why exactly he comes to the Hydra's aid.
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Chimera
Carcinus
Echidna Beast Mode Attempt 1
Medusa