Nereids are sea creatures in Greek mythology. They are the 50 daughters of the sea-god, Nereus and the sea goddess (and half-sister of Nereus), Doris. They are beautiful women who are friendly to sailors and are said to have the powers of prophecy. They live in the Mediterranean sea and are often depicted riding a dolphin or seahorse or another sea creature. They are also in Poseidon’s retinue, which we see reflected in astronomy. Nereid is the name of the third largest moon for the planet Neptune (the roman name for Poseidon).
This is a good site with a lot of information including where Nereids can be found in literary epics such as the Iliad and other mythological sources. http://www.theoi.com/Pontios/Nereides.html