The first reference to Planet X was made by Percival Lowell, and he dedicated the years from 1909 through 1915 to finding it. This was in the beginning of the 20th century, before Pluto was discovered. Some of the pictures taken from his private observatory even showed Pluto on them !

Then Tombaugh found Pluto, as described in the previous writeup.

In 1977-1984 Charles Kowal performed systematic search for undiscovered bodies in the solar system. During his search, Kowal found 5 comets and 15 asteroids. Kowal also recovered 4 lost comets and one lost asteroid. Kowal did not find a tenth planet, and concluded that there was no unknown planet brighter than 20th magnitude within 3 degrees of the ecliptic. Since then a belt of asteroids, the Kuiper Belt has been found outside of Pluto.

The Pioneer and Voyager spacecrafts has revealed nothing, and nowadays Planet X is just history within astronomy.