Shirahoshi

Princess Shirahoshi, known as the "Mermaid Princess," is a giant smelt-whiting mermaid and the youngest of King Neptune's children. She was first mentioned by Pappug, claiming he knew her and even promised to introduce her to the Straw Hat Pirates when they arrived at Fishman Island (though later this is revealed to be a lie). First Appearance: Chapter 612. She is the Ancient Weapon: Poseidon. (Source: One Piece Wikia)