We always use mutagens, but are we really the only ones in the world that can use them?
No mad scientist? No wannabe-mutant? No falied mutant experiment? No really crazy psychopath?
We already have the broken cyborg and the infected survivor, but why not a mutated freak? The chaos? The survivor? The prototype?
Something that just like and Apex Predator, just want to eat you, something that mutate to a point that it can no longer think properly.
Or mutated B.O.Ws with a CBM installed in their brains to force them to fulfill their duty. Kill all remaining humans.
What about a monster for every mutation category?