Carcinus

It was a giant crab which came to the aid of the Hydra in its battle with Heracles at Lerna. The hero crushed it beneath his foot but as a reward for its service the goddess Hera placed it amongst the stars as the constellation Cancer.

Killed by: Heracles

Related image
Heracles and the Hydra. Black-figure hydria, c. 525 BC, from Caere. Getty Villa.