Source: The Mysterious Island
Location: A Mysterious Island
Threat Assessment: 3. Being a giant crab about 15 feet across means you got claws baby. You are also pretty well armored.
Limitations: Animal intelligent, somewhat clumsy, tasty.
Personal Note: My favorite story involving killer crabs was a short Russian one, the title I don't remember. There on an island they released a small crab robot designed to find metal and form it to parts to make another crab. After a few days there was an accelerated evolution as some crabs grew small and fast, and others big and strong. Shame the head scientist had metal fillings in his teeth.