Oh, and that orbit...hmm.

In addition to an unexpected mass, Pluto also had an unexpected orbit.
It's orbit takes it from 30 AU to 50 AU.
It's inclination to the ecliptic is quite large - 17 degrees.
The average size of orbit, the "Orbital Semi-Major Axis" (OSA) determines its period.
Pluto is actually in a 3:2 stable resonance with Neptune.

In addition to an unexpected mass, Pluto also had an unexpected orbit.