[Purulent Carcass]

The entire asteroid belt is believed to never have been allowed to form into a planet, due to Jupiters gravitational effect on the chunks, not allowing them to fuse together as they impact each other. Supposedly the impacts are too violent and because of this, the region lost most of its mass in the early history of our solar system, with the 99.9% of majority getting jettisoned out.

Kuiper and theoretical Oort cloud chunks seem to also be remnants and part of that proto solar system cloud. Several of the larger planet's moons are believed to be from the Kuiper. I think what actually makes Pluto rather unique is that fact it has Charon as a satellite. Eris is bigger than Pluto and they decided that instead of adding a growing number of planets to the roster, they would redefine what it means to be a planet. It pretty much boils down to it needing to be located between the sun and Kuiper belt. Clearing the orbit of planetary neighbors is one of the requirements needed to be classified as a planet, yet Earth, Mars, Jupiter and I think Neptune all have asteroids in their orbits. So there are discrepancies in that whole mess.