So far, this is my interpretation of the evidence; Nibiru could be either A: Jupiter, or B: the Moon, Nibiru was dubbed as the planet of the crossing by the Sumerian civilization around 5000 BCe, which was one of their Gods. The belief principles in Sumer/Babylon around that time were very similar to the later Greek beliefs, I read it somewhere that it states the Greeks and Sumerians were one in the same, or the "original" Greeks came from Mesopotamia, But I'll take that with a grain of salt I think! Today, without much effort, or visual aids one could watch Jupiter "cross the sky". I'm sure the Moon would have been even more intriguing to the ancients with it's cycles, and eclipsing of the sun, and it's visibility during the day, etc. Planet X is Eris.
Nibiru and Planet X are not the same thing.
Now, the term Nibiru could mean anything, like maybe an event that happened once every generation, or a comet, Massive Solar flares( the Chinese depict Nibiru as a dragon chasing a globe). I can dig up more info, but for now it will have to wait until another post.