Princess Shirahoshi

Princess Shirahoshi, also known as the Mermaid Princess (人魚姫 Ningyo Hime?), 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 the Straw Hat Pirates to her when they arrived on Fishman Island (though later this is revealed to be a lie).
She is also the current form of Poseidon.

Shirahoshi is a giant-sized mermaid, approximately the size of her pet Megalo, and is described as "cute but huge" in her introduction; this makes her roughly eight to ten times the size of Luffy, an average human, excluding the length of her tail; she can hold and hide Luffy in one hand alone. According to Caribou's estimation, he put her about seven to eight times the size of a human-sized mermaid.[6] Even as a baby, she was larger than her mother and brothers.