From its discovery in 1930 up until 2006, Pluto was considered the ninth planet in the Solar System. After the International Astronomical Union revised and solidified the definition of a planet, poor Pluto was demoted to "dwarf planet."
No, Pluto should not be considered a planet. The decision came as a result of the vast differences between Pluto and other planets -- Pluto has an irregular orbit, is non-spherical, and is much smaller than other planets. Further, it is likely that a number of objects in the Kuiper Belt are as large as Pluto -- at least one has already been sighted. Rather than labeling every object of this size a planet, a more stringent definition of what a planet is ensures better classification of astral objects.