Tournament of Champions

The Tournament of Champions was an illegal arena circut below Kaon in the days before the great war. Megatronus fought many of his gladitorial victories there.

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 alt-mode they wished. Megatronus allowed this cowardly way of fighting only because it increased his appearence of invincibility when he inevitably won.

Exodus: The Official History of the War for Cybertron

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.