When would this Planet 9 have passed close to Earth? 10,00 years ago? 20,000? 50,000? Would any humans present at the time, if any, had maintained legends about it in some form?
Alas, not close enough. :^)
Given the short duration of observed fluctuations to the orbits of outer gas giants, and the tiny size of Pluto, I've speculated for years that a large unknown will probably be found in an orbit more or less perpendicular to the ecliptic.
Harrington & Van Flandern posited a 2-to-5 Earth mass body which was subsequently/supposedly refuted by Voyager data (oh well) but a perpendicular orbit provides something of an alibi for their model. :^)
It also happens to imply an explanation for the odd Uranian system, which has a parent body with its rotational axis nearly in the ecliptic, but a system of moons which appear more 'normal' than the other gas giants.
By contrast, Neptune's moons are really screwy here and there, while the planet is relatively tranquil and workaday.[Planet Nine: Alternative Hypotheses | Wikipedia]