Is Pluto an asteroid?
No. It is a dwarf planet.
Pluto and the Kuiper Belt objects may share structural similarities with asteroids, but the ones we can observe are much larger than Main Belt asteroids.
Pluto is usually farther from the Sun than any of the eight planets; however, due to the eccentricity of its orbit, it is closer than Neptune for up to 20 years out of its 249 year orbit. Pluto crossed Neptune's orbit January 21, 1979, made its closest approach to the Sun on September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This close orbit will not occur again until September 2226.