Unicron

Chaotic Evil brother of Primus, huge planetoid fuel by devouring planet, literally the embodiment of chaos and can transform into a colossal robot Unicron is mostly a loser when he appear on the screen as he lost to Primus is all powerful energy form, was banished by Primus first children the 13 original Prime, by Rodimus & Optimus Prime. He also got his butt defeated by Galactus. Fun fact, he is often compared to an unicorn.

Ratchet: Unicron is the embodiment of chaos and dark energon is supposedly his blood.