Most astronomers agree that the once-hypothesized Planet X does not exist. There's no evidence for its existence; it has not been seen with telescopes nor have astronomers found any of the gravitational effects that such a body would have on bodies known to exist.
Today, Planet X often serves as a stand-in term for any potential extraterrestrial body located in or near the solar system, much as it did in the late 19th century when astronomers studying gravitational perturbations of known planets coined the term as a stand-in for any unknown body causing such perturbations.
Much of the hubbub about Planet X today stems from sensationalized stories perpetuated by the media and numerous websites that base theories of its existence on bad science or outright falsehoods. One popular meme is that NASA found and announced the existence of Planet X in 1983 after astronomers held a press conference during which they stated that observations in the infrared light spectrum suggested, among other possibilities, that a 10th planet might exist, and then some elements in the media misrepresented the astronomers. Further studies showed that the earlier observations stemmed from gravitational effects generated by other galaxies and by a large gas cloud in our galaxy.