Planet X, or Nibiru, was supposed to explain the oddities in the orbits of Neptune and Saturn. It was supposed to be a gas giant that orbited backwards way out by Pluto(which was discovered when they went looking for planet X) This is actually a fairly old myth.There are some people who think its a brown dwarf companion of the sun that brings death and distruction in cycles on its orbit.
Andy Lloyd's Dark Star Theory Brown Dwarf Binary Companion Nibiru