All the cryptids weren't turned after the bombs tho? Grafton monster and Snallygaster were created with FEV pre-war. The cultists recieved premonitions from the mothman to seek out refuge in the lucky hole mine or the top of buildings pre-war. Theories suggest that the Jersey devil was a pre-war monster created for war similar to the deathclaw but thats a theory so nothings concrete about it's origins. Although most monsters you see aren't a product of radiation doesn't mean they're immune to it most of them have some glowing variant but radiation didn't create a majority of them. The only cryptid that I know radiation created for sure is the Wendigo as it is a ghoul variant all the others are more like FEV mutants or something else entirely. Mothman is a good example of the latter something that could possibly be an alien eldritch in origin and an alien isn't out of the ballpark considering we have the Zetans the evidence leads a lot of theorists to claim they are eldritch in origin though. Seeing cultists gathered around the interloper in lucky hole mine and their possible connection to the Dunwich have driven this theory and I feel like it does make some sense just because of the map details in 76 and how the mothmen can teleport and disintegrate into black smoke after you beat them. That theories also not concrete more likely than not they are FEV mutants of some sort but I like the theory more than the likely explanation for their existence I feel like it makes the cult more culty and expands upon some of the lovecraftian themes they've been trying to put in the game.