Lord Amazon of the Jungle
Plantera
Considering that, in other fantasy genres, Light and Darkness are always having a right scuff at each other. The traditional Light vs Darkness thing.
So, would it be a lot more helpful if the Crimson/Corruption and the Hallow monsters actually infight with each other rather than having ALL of them pick on the lone wandering player who happens to be just passing across their turf.
It might not have to happen all the time, just when both monsters happen to cross each others' paths and set their wrath on each other shortly after. Besides, it would do me world a lot better.
As for the others, the Jungle, Snow and Desert inhabitants, they don't have to beat each other up though.
Besides, it'd be a lot more entertaining to spectate the carnage from the sidelines...and maybe bet Platinum Coins on which side will triumph.