Here's the breakdown:

* Planet X is a hypothetical planet that was proposed in the late 19th century and early 20th century to explain gravitational anomalies in the orbits of Uranus and Neptune. The idea was that there was a large, undiscovered planet beyond Neptune influencing their orbits.

* Pluto was discovered in 1930, but it was never referred to as Planet X. It was named after the Roman god of the underworld.

* Pluto was initially considered a planet, but in 2006, the International Astronomical Union (IAU) redefined the definition of a planet and reclassified Pluto as a dwarf planet. This decision was based on Pluto's size and its lack of dominance in its orbital region.

Therefore, Pluto was never called "Planet X." It was simply called Pluto, and later reclassified as a dwarf planet.