There are 4 species of horseshoe crabs that still exists today. One, Limulus Polyphemus can be found on the east coast of North America and in the Golf of Mexico. The highest concentration of limulus polyphemus is the Delaware bay area although we have a strong population of them here on the space coast in Florida.
Another species, Tachypleus tridentatus which are found in the Seto Inland Sea. (This one is considered endangered, although with regulation it seems to be making a come back.)
The third species is Tachypleus gigas is found off the coast of India.
The forth species is Carcinoscorpius rotundicauda and is also found off the coast of India.
There is one horsehsoe crab species that is extinct and that is Lunataspis aurora. This is also the oldest of the species at about 450 to 500 million years.