Deathclaws in Fallout were created by the pre‑war United States military as genetically engineered bioweapons sometime shortly before the Great War of 2077 in the series’ timeline.

In‑universe creation

  • Deathclaws were designed by the U.S. government using mixed animal DNA, with Jackson’s chameleon as the primary base, to replace human troops on the battlefield.
  • This military bioweapon program took place in the years leading up to the Great War of 2077, so by the time the bombs fell, early versions of deathclaws already existed in labs and controlled facilities.

After the Great War

  • When the nuclear war devastated civilization, many of these experimental creatures escaped into the wasteland, where they spread and became one of the most feared post‑war predators.
  • Over time, various factions and experiments (such as later Enclave and “refined” variants in some lore sources) produced more intelligent or specialized strains, but all trace back to those pre‑2077 U.S. military projects.

TL;DR: Deathclaws were not a natural mutation; they were engineered as military bioweapons in the years just before the Fallout universe’s Great War of 2077, then escaped and proliferated in the wasteland.

Information gathered from public forums or data available on the internet and portrayed here.