Ammo Knights

The Ammo Knights serve as the Royal Guards for the Neptune family on Fishman Island. They wear large striped snail 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.

History
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.

Trivia

 * Ammo Knight is a play on the word "ammonite", the fossilized remains of the shells from snails or snail-like organisms, given that the knights wear such shells as part of their uniforms.