When pluto was discovered, it was thought to be atleast as big as earth; big enuf to have an effect on the orbits of it neighbours. But the reality came out that it is only 1/6th of our Moon.
Pluto is neither a planet nor a satellite so it is a dwarf planet. Pluto dint fit the pattern set by the major bodies in the solar system
It is closer to being a humongous comet, rather than a planet.