Nature really likes crabs and keeps trying to evolve more of them.

Carcinisation is the name of a process of convergent evolution, in which crustaceans evolve into crab-like forms. In decapod crustatceans, there have been at least 5 distinct times when crab-like animals have evolved.

Coconut crabs, king crabs, and hermit crabs are not types of true crab, and are only distantly related.