Dwarf planet Sedna, which is about 3 times as far as Neptune, is a mix of water, methane, and nitrogen ices with tholins. Its orbit takes 11,000 years to complete - pretty amazing, huh?
We don't actually know how it originated either. Could it have been captured from a nearby star system? Is there a larger planet beyond the orbit of Neptune and Sedna is its moon? Go ahead and look that up; some of the theories are pretty interesting. While you're at it, explore some of the other dwarfs of our solar system.