The marble and stone basins of Fontana del Nettuno and the nearly identical Fontana del Moro at the south end of Piazza Navona were created in 1574, but only Fontana del Moro had statues from the beginning. The Neptune fountain had no statues for over 300 years, until they were added in 1873-78. The statue of Neptune fighting a giant octopus was sculpted by Antonio della Bitta, and the remaining statues including the mermaids (nereids) were sculpted by Gregorio Zappalà, depicting a scene from Greek mythology.
For more images and data on the Neptune Fountain, go to this page dedicated to this fountain and its sculptures.