From Nookipedia, the Animal Crossing wiki
The Horseshoe Crab (カブトガニ, Kabutogani) is a type of deep sea creature known as a Benthic, it is found at the bottom of the sea in Animal Crossing: New Leaf. It is indicated by a circular shadow, and diving is the only way of capturing it.
Donating to the Museum
In New Leaf
After donating the Horseshoe Crab to the museum, it can be found in a large pool in the first room of the Ocean exhibit, along with a number of other deep sea creatures. The exhibit has this to say about the Horseshoe Crab:
"Horseshoe crabs are called "living fossils," as they have not evolved since the age of the dinosaurs. Their back are covered by a hard shell, forming a simple silhouette with their flat backs and long tails. Though referred to as crabs, they're actually more closely related to spiders and scorpions.