Triton

Triton is a moon orbiting the planet Neptune. It is the planet's only spherical and actually big moon.

History
Triton probably formed along with the other large dwarf planets, Pluto and Eris. It's larger than Pluto, so it's the largest object in the kuiper belt at that time.

Its orbit was so close to Neptune that neptune's gravity was enough to pull Triton. When Triton slowly entered orbit around Neptune, it was spinning the wrong way, making its orbit retrograde. It also bumped some of neptune's original moons out of orbit, made some crash into others, and made some crash into Neptune or Triton.

Neptune's old moons that have broke apart formed a big ring of rubble. That ring formed Neptune's inner moons and Proteus. The only large moon of the original set of neptune's moons that survived Triton was Nereid due to its orbit being very far away from Neptune, but after Triton entered, its orbit became very eccentric.