It's an arthropod, a very, very evolutionarily ancient arthropod. They are like cockroach old.
Latin name Limulus polyphemus.
They don't look like crabs, but they do look a bit like horseshoes. They live in the ocean, and come out onto the beach to spawn.
Interestingly, blood from horseshoe crabs is used to detect bacterial components. This assay, called the Limulus Amoebocyte Assay, relies on factors found in horseshoe crabs that respond to LPS (lipopolysaccharide, a component of Gram-negative bacteria.)
I saw a real one up close the other day. There were some people from the Boston Aquarium showing kids stuff at a shopping mall. Horseshoe crabs are deceptively empty. They look like a big bowl, but if you flip it around, you notice that in fact, most of it is just space, and there's very little crab underneath the shell. Kind of weird.