In fact, Megatronus won so many victories in the Tournament that the rules of combat had to be changed just for him. His opponents were allowed to enter the arena first, then scan and assume any altmode they wished. Megatronus allowed this cowardly way of fighting only because it increased his appearance of invincibility when he inevitably won.
After victorious combat against a trio of primitive Shockwave-created combiner warriors, the crowd began to chant Megatronus' name as the shorter, more forceful, "Megatron!" The victorious gladiator liked the new twist on his chosen name and quickly adopted it.