It's not a planet because it's too small to be considered an actual planet, which is why they call it a dwarf planet.

Pluto is now classified as a dwarf planet rather than an actual planet.

"Earth" or perhaps the planet "Terra".

The Roman god (actually goddess) Ceres does not have a actual "planet" named after her. The dwarf planet in the Asteroid Belt is named Ceres.

Pluto is classified as a big ball of gas now, so Pluto is not considered as a planet anymore.