This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Karkinos (Greek)
Cancer (from Latin; sometimes known by the transliteration Carcinos, from the original Greek Karkinos) or, simply the Crab, is a giant crab in Greek mythology that inhabited the lagoon of Lerna. He is a secondary character in the myth of the twelve labors of Heracles.
Hera was watching the fight between Hydra and Heracles, hoping that the monster she had nurtured would kill her foe. Observing that Heracles was getting the upper hand in the battle, Hera dispatched Carcinus to attack and distract the Greek hero, hoping that this would provide a killing opportunity for the Lernaean Hydra.