There are a few aspects of the Fallout series that make it iconic. The campy 1950's era science fiction vibe, the oftentimes comical violence and finally, the various mutations. Alongside the Ghouls and Super Mutants that have been included in each installment of Fallout, we've been terrorized by the inclusion of the Deathclaws.
Originally developed as replacements for infantry soldiers before the Great War, the Deathclaws are the result of several bouts of genetic engineering. Decades after the Great War, the Deathclaws were discovered by The Master and tinkered with, before being found by the Enclave a century later and tampered with once again. By the time Fallout 4 comes around we've seen various stages of the Deathclaws evolution, but their core identity remains the same - they're apex predators.
Designed to hunt and kill on a battlefield, the Deathclaws use their superior strength, speed and endurance to dominate the wastelands of the United States post-Great War. In the initial decades after the Great War, around the time of Fallout 1, they were seen as a creature of mere myth and legend. As the timeline progressed however, their existence was accepted and they became little more than just another deadly creature that wandered the wasteland.
By the time we get to explore California in Fallout 2, the Deathclaws have been tampered with by the Enclave to become more intelligent. Why you'd make a lethal killing machine more intelligent, I have no fucking clue, but they did it anyway and so the Deathclaws developed the ability to talk. They don't have vocal cords though, so they're just mimicking the sounds that we humans make, which was a nice little detail to throw in there.
The fact that Deathclaws should logically be the dominant species in the wasteland by almost any standard is made a little more palatable by the fact that they can't actually talk like we can. These featherless parrots may outclass us in pretty much every other category, but hey, at least we've got functioning vocal cords. Although, I'm not certain how comforting that would be while a brood of Deathclaw hatchlings is gnawing on your innards.
There's a hairy variant of Deathclaws around the Chicago area, apparently they developed this fur to better cope with the cold. These one's developed the ability to mimic human speech as well, so maybe that's a trait that all Deathclaws are capable of developing under the right circumstances. The main issue with the Hairy Deathclaws is that everyone points out that Deathclaws are still reptiles, and reptiles are cold blooded so fur wouldn't do anything to help them. Again, I'm not sure how comforting that would be while a Hairy Deathclaw is usingyour corpse like a Tauntaun to incubate its eggs.
The simple fact is that Deathclaws are towering mutants that were genetically designed to attack tanks on foreign battlefields. If they became sluggish at night because they accidentally didn't bask in the sun for long enough then they wouldn't be that big of a deal. Ever had a Deathclaw attack you at night? Pretty terrifying, right? Yeah, let's just assume that the genetic engineers found a work around for the cold blooded aspect of their genome.
Whether by natural processes or by direct human intervention, the Deathclaws have adapted to life in the wasteland. Why they don't absolutely dominate the entire continent, I'll never know. Maybe their birthrate isn't high enough, but that seems unlikely. I get that ever since Fallout 3 they've become little more than dangerous monsters for us to kill, but within the lore of the Fallout Universe the Deathclaws should be a much bigger threat than they currently are.
The only thing that set the ancient humans apart from their more ferocious animal neighbors was their intelligence. I guess you could say that our opposable thumbs played an important role in our advancement, but Deathclaws have those as well! The point is that we were slow, squishy and we had fewer sharp or pointy bits, but we were able to think in a much more complex manner than anything else around us, and thus we dominated.
If those same reasoning powers were given to a 10ft tall lizard monster that's covered in scaly armor, claws, horns and teeth... we would have a serious fucking problem.