Naiad

The naiads are creatures that live in Poseidon's realm the sea. Charybdis, was once a beautiful naiad as well, although she was turned into a monster because Zeus was angry that she was taking so much land.

The Titan's Curse
While Percy and Zoë were in the canoe together, Zoë told Percy that the naiads were cursed and that they had never forgiven her, maybe because she had become a Huntress instead of staying with the Hesperides.

Battle of the Labyrinth
In the Battle of the Labyrinth, there was a naiad that had helped Percy to try to clean the flesh-eating horses' stables, the naiad reminded Percy of Rachel Elizabeth Dare, because she was asking so much questions, Percy felt like she was throwing rocks at him.