Deathclaws being genetically engineered before the Great War and later refined by the Master is mentioned in the Fallout 2 Official Strategies & Secrets strategy guide, written by Fallout 2 lead designer, Matt Norton, as well as in Fallout Tactics. It is unknown why the Master would genetically manipulate the deathclaws and then not use them at all for his purposes.
It has been said that they are a mutant Jackson’s Chameleon. The deathclaws were originally created before the Great War by the government
to replace humans during close-combat search-and-destroy missions. They
were derived from mixed animal stock and then refined by the Master, using genetic manipulation. The resulting creature is almost unbelievably fast and powerful.
deathclaws were still mysterious and legendary creatures to many
inhabitants of southern California. They could only be found in the Boneyard at the warehouse, with a mother living there, one in a former deathclaw’s nest and around the Hub,
sometimes attacking caravans or travelers. Later, as the deathclaw
population grew and really spread across the continent, the deathclaws
stopped being perceived as legendary beasts, and started to be seen just
as another dangerous creature.
Deathclaws have a hunchbacked, bipedal reptilian build with long
humanoid arms. They stand roughly nine to ten feet tall, with a thick
and resilient hide, powerful muscles, and twelve-inch-long, razor-sharp
claws that can kill almost any other creature in only a few swipes;
hence their name. They have an excellent sense of smell and hearing,
though their eyesight is poor. Their build gives them incredible speed,
agility and strength in close combat, making them an extreme threat.
Though they were originally mutated chameleons, they have lost the
ability to camouflage themselves.
Deathclaws are very aggressive, territorial, and carnivorous. They
typically live in small packs of around eight to twenty members led by
an alpha male, who is the only one allowed to mate with the females.
Deathclaws are oviparous, with the eggs typically laid in dark,
sheltered locations and fiercely guarded by their mother. Young
deathclaws reach maturity under the care of their parents, following
them closely until they are old enough to fend for themselves.
Interestingly, deathclaws do not sharpen their claws on nearby objects,
instead preferring their own nails to do the sharpening. The Lone Wanderer or Courier can witness this behavior while sneaking up on a deathclaw.
tl:dr - They were Jackson’s Chameleon