Pluto

Poor Pluto! Once it was classified as the solar system's ninth planet. Now astronomers have decreed that it is no longer a real planet, just a Dwarf Planet.
Pluto has its moon Charon to blame for its change in status; one of the rules an object must pass to be classified as a planet is that it must have cleared a path of all "large objects." Charon, compared to Pluto, is much too massive to be considered a "small object."
Charon is large enough that as Charon and Pluto orbit each other, the center of that orbit does not even lie within Pluto.
Pluto is sometimes outside of Neptune's orbit, sometimes inside.
Pluto's mass is less than one percent of Earth's mass.
Pluto is smaller than seven of the solar system's moons, including our own. The other six are Ganymede, Titan, Callysto, Io, Europa and Triton.