Pluto is so much different from the giant planets that since Clyde Tombaugh discovered it in 1930, various hypotheses about the origin of Pluto have been put forward many times.
It is known that the position of Pluto with the solar system contradicts the empirical rule of Titius-Bode, which predicts for him the semimajor axis of the orbit of 77 AU(With a real value of 39 AU). Neptune does not have a good match either: 30,1 AU Instead of the predicted 38,8 AU. But the position of the planetary orbits is in fact determined by the theory of resonances, and the Titius-Bode rule is its particular case. The position of the orbits of Neptune and Pluto corresponds to 1: 2 and 1: 3 with respect to Uranus and, as a result, 2: 3 for the orbit of Pluto relative to Neptune.
In 1936, when they did not yet know that Pluto was a double planet, a hypothesis was proposed that once was one of the satellites of Neptune, but as a result of rapprochement with an unknown planet it was thrown out of the system, and another Neptune satellite – Triton – moved in this case to an unusual orbit with reverse rotation. It was even supposed that, based on the current orbits of Pluto and Triton, you can calculate the mousse and orbit of an unknown planet.