Mutants
Twisted creatures brought into being by the apocalyptic mega death of the Old World, Mutants (or Muties as some call them) are creatures who's DNA has been dramatically changed by the vast quantities of hazardous fallout left behind by the devastating weapons that caused The Fall. With their genetic code irreversibly damaged (though some might say improved) these poor souls rapidly evolved and devolved into ever changing shapes and forms, often with fatal results. Those "lucky" enough to make it through this gruesome process would go on to not only survive, but to thrive in the Wasteland, being granted an evolutionary advantage over their Norm cousins. Having been forged in the hellish genetic foundry of Skydark, they were now highly resistant to the destructive power of radiation and could venture into all but the hottest of rad pits. Combine this with any number of their exotic and freakish traits, it is little wonder that the Mutant population exploded exponentially over a short period of time while Mankind could only just claw his way from the rubble. As existentially threatening as this may all sound to the Human race, it's thankfully more nuanced than that. To describe Mutantkind as a singular entity with a unified goal to wipe all unmutated life would be patently false. While there do exist intelligent subspecies of Mutant, they are often simple minded and tribalistic, fighting among themselves as much as their Norm cousins. And while some Mutants are leviathan in size and power, it is far more common to see random variants of mutated flora and fauna which are just reacting to animal instinct and not malice (usually). That isn't to say Mutants aren't a serious threat, and many an unsuspecting victim has died screaming to their slavering jaws. Like raiders, rads, or thirst, Mutants are just another obstacle you must overcome while surviving in The Wasteland and until the chems and rads dissipate entirely from this ravaged world we're stuck with them.