Pluto does not account for the orbits of Uranus and Neptune, which is interesting because I think I was told in grade school that Pluto was found after a prediction was made that something should be out there. However, they neglected to tell me that Pluto wasn't it after all.Neptune was discovered by looking at perturbations in the orbit of Uranus. (I'm sure there is a dirty joke in there somewhere.) People originally started looking for Pluto because of small perturbations in Neptune's orbit. Percival Lowell hypothesized that Neptune's orbit was being perturbed by an as-yet-undiscovered ninth planet which he referred to as Planet X. Lowell's second predicted location of Planet X resulted in the discovery of Pluto. Percival Lowell is why Pluto begins with P-L.
The funny thing about this is that Lowells was wrong. Pluto's orbit does not effect the orbit of Neptune. The unexplained perturbations in Neptune's orbit are because the initial estimates for Neptune's mass were incorrect. Finding Pluto was actually just dumb luck.
You have wikipedia and my boredom to thank for this little bit of storytime. Unless all of this was made up by some bored teenager in Kansas or something.