The Ammo Knights serve as the royal guards for the Neptune Family on Fish-Man Island. They wear large striped spiral shells as helmets and shoulder pads with their eyes covered by a black mask of some kind. The helmets have a spike sticking out in the front, similar to a that of a unicorn. They carry tridents with them.
The Ammo Knights are presumably strong being the royal guard of Fish-Man Island. They are powerful enough to maintain law and order in the kingdom though they were no match for the New Fish-Man Pirates, although the pirates needed energy steroids to win implying that the Ammo Knights are stronger than normal Fish-Men.
They are first seen when the princes visit Mermaid Cove. They are ordered to apprehend the Straw Hats around the island but fail to do so. Later, at the palace, they are defeated by the Straw Hats and get tied up along with Neptune and his ministers.
Later, several Ammo Knights appear outside the island and inside the control center for the island's air tank, ready to help Luffy and the princes launch a burst of air at Noah to redirect its course.