Theoretically, it's a moon that got out of Neptune's orbit. It might have been a moon, but now, it's an icy rock, with a moon that's near half the size of it orbiting it, Charon.
I think that anything that is in the orbit of the sun that is not plentyful( aka asteroids) should be a planet. that being said pluto and it's moon actually orbit around each other aswell as the sun so i don't think it should be a planet