There is a lot out there in the wilderness of the internet on Nibiru or Planet X:
This planet will, these sites say, cause cosmic disaster. The problem is, that any large planet close to the Earth can be detected by either telescopes, or, if it is hiding peek-a-boo behind another close planet, detected by gravitational effects. However there may be a Neptune-sized planet out beyond Pluto, but there is no empirical evidence of this, as yet.
It probably does not matter one way or the other, since we have more than enough problems on this globe to deal with: