The horseshoe crab is one of the most ancient creatures in the world. Many people think of dinosaurs as being old, but when dinosaurs roamed our earth, the horseshoe crab had already existed for one hundred million years. In fact , it looked then, much like it looks today. Horseshoe crabs haven't changed very much in more than 300 million years. That's why horseshoe crabs are often  called living fossils.