Fun fact: horseshoe crabs are not only not crabs (infraorder Brachyura), not only are they not decapods (order Decapoda), and not only are they not class Malacostraca, they're not even crustaceans (subphylum Crustacea).
Instead, they're in the subphylum Chelicerata, making them more closely related to a tarantula, a daddy longlegs, and a tick than they are to any crab.