All the planets lie along the plane of the ecliptic. The fact that Pluto doesn't discounts it from being a planet. Any other body that orbits the Sun and who's highly elliptical orbit comes within the orbit of Jupiter will, at some point in it's lifetime be affected by Jupiter's gravity.
Why do you think there aren't any objects larger than the smallest planet (Mercury) zipping about the Solar System any more? Because in the earliest time of the Solar System's formation, such large bodies were dealt with by the other large bodies (the Sun and planets) that were forming.