I come at this from the perspective of a person who is 99% convinced its absurd, so take what I say as you will.
As I understand it, Nibiru, though it sometimes has other names, is another supposed planet in the solar system, relatively large, who orbits in a rather elliptical pattern, that periodically passes close to the earth, or maybe collides. Supposedly, in the near-ish future, it will make a near pass with the earth, and likely bring an end to civilization as we know it.
In principal, this jives with orbital mechanics. if we believe that its on a very large, very slow orbit, its possible that it would have passed once, long ago, been observed, and be able to pass by again. There are comets with similar orbits, generally not planet sized, but perhaps we can entertain the idea that its possible some satellite of Jupiter, perhaps, was thrown out into a very large orbit at some time in the distant past.
Nibiru, from what I can tell, is supposed to have an orbit somewhere between several thousand years, and several hundred thousand. If we are to believe that it was observed by perhaps the Maya, or the Sumerians, as some sources seem to claim, it has to be on the range of a few thousand at most. Though many ancient cultures had reasonably developed astronomy in some respects, it seems a strain to suggest that anyone prior to the last few centuries could make reliable predictions of an objects orbit based on only one sighting. Halley's comet was likely long known because it came back reliably, every 76 years, if instead it came back every five hundred years, it would have taken at least 1500 years to figure out it was the same object, and to make predictions about when its going to show up again.
so: its unlikely that anyone predicted Nibiru to come long ago, but that doesn't mean its not coming now. If it has an orbit on the longer side, say, 500 000 years, its not unreasonable that it never would have been observed by humans until quite recently. We could characterize it as a very large long period comet, flying past on its single, or simply very rare visit to the inner solar system. The fact that its large would make it easier to see long before the close pass with the earth, which is essential if we can predict that its going to make such a pass to begin with, and not more than say, a few days or weeks before it happens.
While this is true, an easily observed Nibiru poses a number of difficult problems. NASPost too long. Click here to view the full text.