I have a world build for an Eastern style RPG, which includes the standard trope of monsters being common obstacles the heroes have to battle across the world; though my world gets into detail about where the monsters came from and integrates them a bit more into the actual world and story than many RPGs.
The backstory of the world involves humanity’s first use of magic ending poorly when the magic 'ran out' from overuse, and a side effect of how it was used at the end, when they were desperate to find any way to sustain their spells, was the creation of a sort of magical pollution. This magical pollution acts as a mutagen, mutating human, crops, and animals. In the case of animals, this is what created the monsters of the world.
'Mutated' humans also happened, though far less often than animals (our closer connection to magical energy helped to protect us slightly). Most mutated humans resulted in miscarriage or still birth. A few were born alive but with 'mutations' of some form:
- most of these mutations being disabling,
- a few making the human look different but not otherwise disabling them, and
- a very few rare humans were born (somewhat) stronger then the average humans or otherwise had 'positive' mutations, though not so strong as to make them superhuman or medieval X-men.
This magical pollution dissipated rapidly, and a few generations later the decimated humanity started to recover from the original disaster. By the time of my story the background pollution is still around, but in a much lesser form. Monsters have grown weaker as a result, with stronger monsters generally being in areas where this magical pollution would be more common. However, monsters are still present, and generally have a few 'common' forms in a region. The basic idea had been that the monsters produced at the maximum of the pollution kept breeding and spreading so that they stuck around despite new monsters not usually being born any more; but still feeding off this magical pollution as a source of strength (justifying why decreased pollution tends to weaken them and generally why their energy output is higher than their energy input from hunting should justify).
I realized this was a problem once I threw in humans. I don't want 'mutated' humans to be having children that are similarly mutated. In general I want these magically spawned differences in a human to not pass on to their children. Occasionally a 'mutated' child is still born in present day, but they’re rare and treated as such (varying areas ranging from shunning them as 'evil' to simply treating them as a birth defect to be pitied).
So how can I justify the fact that the monsters are still sticking around, even if weaker than they were when the magical pollution was strongest, but that the effects on humans did not get passed on?
I don't want to fall back on the claim that 'mutated' humans failed to ever have children. Koinophilia would definitely be present, but at some point at least a few of these individuals will find a spouse. Besides, for plot reasons the story works better if these 'mutations' are not generally passed down to children.