In addition to actual objects that have just been demoted from planet status there has been multiple supposed planets astronomers have been trying to locate throughout history.
Vulcan was a speculated planet that was expected to reside between Mercury and the Sun. The perturbations in Mercury's orbit suggested there should be something there. These anomalies were however later explained by Einstein's theory of general relativity.
Counter-Earth is a speculative planet that would reside in the same orbit as Earth, but at the opposite end of it. This our supposed twin would be near impossible to detect due always being behind the Sun.
According to some theories Earth and Moon were formed after the planet Gaia (pre-earth) was split in collision with another planed named Theia. It might have been an icy dwarf planet originating from the Kuiper-belt and much of the Earth's water came from there.
Another theory of the forming of Earth, as well as the other inner planets suggest two super-Earth planets Enyo and Bellona once were the only planets inside Jupiter's orbit. The collision of these two would have resulted Enyo being ejected from the solar system, leaving behind its moons. These moons included objects that then became Mars, our Moon, Pluto and some other dwarf planets. Other inner planets and the asteroid belt were formed from the remnants of Bellona.
The existence of asteroid belt suggest that there might have been a planet in between Mars and Jupiter. All those asteroids would be the debris from the destruction of this planed called Phaeton. This shouldn't be mixes with Planet V which could have been residing between Mars and the asteroid belt. The close proximity to the asteroid belt eventually altered it's orbit leading it into a collision course with the sun.