Which hypothetical astronomical body was known as Phaeton?

Phaeton (or Phaëton or Phaethon) is a hypothetical planet believed to have existed between the orbits of Mars and Jupiter. The destruction of the planet supposedly led to the formation of the asteroid belt.

A German astronomer, Johann Daniel Titius, believed that an undiscovered planet existed between the orbits of Mars and Jupiter. Another astronomer, Johann Elert Bode, urged for a search for this unknown planet. In 1801 Giuseppe Piazzi discovered an orbiting body in the right position, it turned out to be Ceres, the largest of the asteroids. In 1802 a second body was discovered in the same general orbit, the asteroid Pallas.

The astronomer who discovered Pallas, Heinrich Olbers, suggested these orbiting bodies where remnants of the planet, and named by linguist Johann Radlof "Phaëthon" of Greek mythology. Phaëthon, the son of the sun god Helios who attempted to drive his father's solar chariot for a day with disastrous results and was ultimately destroyed by Zeus. The name Phaeton came from Soviet astronomer Sergej Orlov in 1949, which became the established name for the hypothetical planet.

The current consensus holds the asteroid belt is the remains of the protoplanetary disk, that was not able to form into a planetary body.