There is no planet x, and they never found one. It was an error based on an overestimation of the mass of Neptune.
“Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell’s quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
Clyde Tombaugh’s discovery of Pluto in 1930 appeared to validate Lowell’s hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus’s orbit were due to a slight overestimation of Neptune’s mass”
Eris is about the same size as Pluto. At the time it was discovered, Pluto was considered a planet. Hence, Eris would be a planet, too. But instead of announcing that they had found a tenth planet, astronomers FIRST voted to change the classification of Pluto THEN announce that they had discovered Eris, by which time they had also discovered Haumea. So there were never big headlines; Eris would be covered like just another asteroid.
Eris was not the explanation for what made some astronomers look for a tenth planet, but that has nothing to do with whether or not it was a tenth planet.
Incidentally, the definition of a planet used to deny Eris and Pluto are planets is ridiculous. It’s very complicated and yet useless as a universal standard for looking beyond our own solar system. A much simpler one would be that any object which is sufficiently large enough to be round to due hydrostatic equilibrium and which revolves directly a primary star is a planet. The complication for that standard is works great for rocky planets, not so great for snowballs.