We have a habit of naming our planetary bodies after Greek and Roman gods. Just in case you missed some days in elementary school, there are 8 actual planets in our solar system, and a variety of asteroids and dwarf planets.
We've already used Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune to name the major planets. All of these are the Roman names for Greek gods, except one. For some reason we decided to use the Greek Uranus, instead of the Roman Caelus. I'm not sure why. Caelus would have prevented decades of butt jokes.
In addition to those 8, we've also named a bunch of moons, dwarf planets, and large asteroids. So far Pluto, Ceres, Pallas, Juno, Vesta, Astrea, Eris, Hebe, Iris, Metis, Hygeia, Eunomia, Iapetus, Rhea, Tethys, Dione, Io, Europa, Ganymede, and Callisto are all taken. Some of these are Greek, some Roman, and some not even gods or titans. In fact there is one moon just named Titan, which isn't even a name for an individual god, but a type of proto-god.
This supposed planet is so remote, and far from the sun that it doesn't reflect light, which is why it hasn't been physically observed yet. Far away, dark and cold, it should have a name that reflects that, and since we already gave Pluto the name of the god of the dead, we'll have to find something else.
Ophion is my choice for a new name. Bare with me, it makes a ton of sense.
According to the Orphic tradition in Greek mythology, sometime between the Titans' revolt when they ganged up on Uranus and castrated him, and Kronos taking power as the King of the Titans and the universe, there was another Titan in charge named Ophion. Kronos fights this other Titan and physically tosses him off of Olympus into the sea. Ophion doesn't appear anywhere else in Greek mythology, and the story doesn't appear anywhere outside of the Orphic tradition. It is speculated that Ophion is another name for Oceanus, who was the Titan of the oceans, and did not participate in the castration of his father Uranus. The theory is, Ophion is cast out of Olympus into the sea, and becomes Oceanus, the lord of the deep, dark, cold world-circling ocean. Oceanus, though, is a stupid name for a planet, especially one that probably doesn't have any water.
It has been theorized that early in the solar system's life, Jupiter and Saturn played a part in ejecting other large gas giants from the solar system. It is also theorized that this Planet X was probably one of those planets to get thrown out, which is why it now exists so far from the sun. If this is true, than Planet X, like Ophion, was physically removed by Saturn (or Jupiter, whatever), and tossed out into the cold space at the edges of the solar system. I told you naming it Ophion made a ton of sense.