The asteroid belt doesn't have nearly enough mass to be/have been a planet[0]. Largest thing there is Ceres, followed by Vesta, of which only the first qualifies as a dwarf planet.
There was an hypothesis about it being the remnants of a destroyed planet[1], but that was mostly an idea to support the Titius-Bode law[2], which was disproved a few hundred years ago.
There was an hypothesis about it being the remnants of a destroyed planet[1], but that was mostly an idea to support the Titius-Bode law[2], which was disproved a few hundred years ago.
[0] Currently about 3x Ceres mass, though it may have about Earth mass early in its history. See https://en.wikipedia.org/wiki/Asteroid_belt#Formation and https://en.wikipedia.org/wiki/Asteroid_belt#Evolution
[1] https://en.wikipedia.org/wiki/Phaeton_(hypothetical_planet)#...
[2] https://en.wikipedia.org/wiki/Titius%E2%80%93Bode_law