Although known by many names, this arthropod is not a crab at all.
Shaped in the basic shape of a horse shoe, it has also been called a horse foot crab, helmet crab and also king crab. The important thing to remember that the “Horseshoe Crab” is more closely related to spiders than it is to crabs.
The scientific name for this horseshoe crab is Limulus Polyphemus. It lives on the eastern coast of North America from Maine to the Yucatan Peninsula and within the Golf of Mexico.
I have received reports though of some that has wandered all the way across the ocean to the British Isles. While I have not verified this for myself, I have had pictures sent to me of one that was found.