How did the leader of the Super Mutants get all messed up in Fallout?

Answer 1

You are talking about the Super Mutant Behemoths. They just have a lot more radiation and it makes them bigger - like the HUGE roaches called rad roaches or (radiation) roaches. However, this was originally an concept from Fallout 1, where the FEV virus was let into the air by an accident by the US Military following the apocalypse. The FEV virus was the original cause for all major mutations in the Fallout world and was so detailed that one can be forced into an super mutant state by dipping yourself in the vats of the Pure FEV (Pure Forced Evolotionary Viruis). Yet, if the original canon is valid in Fallout 3 is still to be told.