How do Deathclaws fit into your definition of "real stuff"?
As for the OPs suggestion... A real werewolf, I don't care for that... But belief that the self turns into a wolf at night is a genuine mental disorder recognized by the DSM IV... Something were the person blackouts (obviously not for the main character) and commits atrocious acts, would work. The main player likely couldn't utilize this kind of perk without introducing a fantaasy element to the setting.
Because Deathclaws arent some random pick of the hat, they were made as an a Pre-War weapon, but it was ultimately a failure. Pretty this was in the Fallout bible, or one of the games. Can't recall where.
Okay to fit into a fallout term why not become a deathclaw for a amount of time as a perk
Deathclaw species
Human Species
there is no cross breed. Fallout is a retro futuristic game, not a Bioshock gene splicing game.