This group of animals is also known as horsefoot, king crab, or saucepan. Some people call the horseshoe crab a "helmet crab", but this common name is more frequently applied to a true crab, a malacostracan, of the species Telmessus cheiragonus. King crab is also more usually applied to a group of decapod crustaceans.
Limulus means "odd" and polyphemus refers to the giant in Greek mythology. It is based on the misleading idea that the animal had a single eye.
Former scientific names include Limulus cyclops, Xiphosura americana and Polyphemus occidentalis.