nereid -
One of 50 sea nymphs who were attendants upon PoseidonNeptune, and were represented as riding on sea horses, sometimes in human form and sometimes with the tail of a fish.
Wikipedia
The Nereids are sea nymphs, the fifty daughters of Nereus and Doris. They often accompany Poseidon and are always friendly and helpful towards sailors fighting perilous storms. They are particularly associated with the Aegean Sea, where they dwelt with their father in the depths within a silvery cave.
