A few months back someone mentioned Planet X on FR and I did a search on it. True or not, Planet X apparently has a rather large following.
A professional astronomer who is a friend of mine has told me that the Planet X theory - although very vague - has support because of the odd behavior of Neptune & Pluto. Those two seem not to follow the pure mathematical rules of motion and it is as if another, unknown, planet was out there influencing their orbits. So Planet X exists on paper as a means of explaining some anomalies. But its "influences" are so vague and unpredictable that it has never been possible to pinpoint its location, much less spot it with a telescope.
I have no clue as to what would be the name of this planet. Considering that it is supposed to influence Pluto, who was king of the dead, maybe Orpheus. Or Mickey Mouse.