Unicron

Unicron the World Devourer was a giant mech capable of transforming from a planet to a huge robot. He featured in the movie that 2k5 is based on, and continues to occasionally make his existence felt. After the events of the movie, Unicron's head was left in orbit around Cybertron. After Cybertron was moved to Alpha Centauri, the head crashed at Magnaron, where it remains today.