When Clyde Tombaugh first discovered it in 1930 it was thought by several astronomers who interpreted irregularities in the orbits of Uranus and Neptune as being caused by a more distant planet X. Among these astronomers was the American Percival Lowell who is credited with the successful prediction of the planet's orbit. Lowell also started the search for the planet which was ultimately found in 1930 by his successors at the Lowell Observatory in Arizona.

However the mass of Pluto is now known accurately since a satellite Charon has been discovered. Pluto's mass is only 0.002 Earth masses, while Lowell required Planet X to have seven Earth masses to produce the effects on the other planets.

So Pluto was discovered quite by accident. It is also claimed that the
perturbations of Uranus and Neptune are actually none existent and caused by
mathematical errors. So there is no reason to even expect a large enough planet out beyond them. Pluto was considered to be a planet because it was expected, and thought to be larger, and where they expected it to be. But since it is actually much smaller, and was only coincidentally in the right place, it has no business being called a planet.