Nereid (moon)
It was the farthest of Neptune's known moons for a long time (Halimede through Neso have since taken that honour), and is the third biggest, with a diameter of 340 km.  Its orbit averages 5,513,400 km in radius, but is very eccentric and varies from 1,353,600 to 9,623,700 kilometres.  For a long time this was the most highly eccentric orbit of any known moon in the solar system, but Bestla, a more recently discovered moon of Saturn has a more eccentric orbit.  The unusual orbit suggests that it may be a captured asteroid or Kuiper belt object, or possibly that it was perturbed during the capture of Neptune's largest moon Triton.  Very little else is known of Nereid.