What is the name of the planet-eating robot in Transformers?

The planet-eating robot in Transformers is Unicron.

Unicron first appeared in the 1986 animated film The Transformers: The Movie. He is a colossal Transformer who can change into a planet-sized mechanical world and consumes planets, moons and other celestial bodies. His opening attack on Lithone immediately establishes him as one of the franchise’s most powerful villains.

Unicron recruits the dying Megatron and rebuilds him as Galvatron, along with new Decepticon troops. He fears the Autobot Matrix of Leadership, but Galvatron fails to control it. Hot Rod eventually takes the Matrix, becomes Rodimus Prime and uses its power to destroy Unicron’s body. Only Unicron’s head remains, later orbiting Cybertron.

A common mix-up is Primus, Unicron’s opposite and often his twin or creator, depending on the continuity. The Quintessons are also important Transformers figures, but they are not the planet-eating robot. Unicron’s origin and powers vary between continuities, while his planet-eating identity remains consistent.