Triton is, with a diameter of 2,700 km, the largest of Neptune’s 13 moons. It is the only large moon in the Solar System with a retrograde orbit (an orbit in the opposite direction to its planet’s rotation), which cannot have formed out of the same region as Neptune, so it must have been captured from elsewhere. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been captured from the Kuiper belt.
Despina is a tiny moon of Neptune. A mere 148 kilometers across, diminutive Despina was discovered in 1989, in images from the Voyager 2 spacecraft taken during its encounter with the solar system's most distant gas giant planet