Deathclaw

Deathclaws are the spawn of government tests on Jackson's Chameleons mixed in with DNA of other species. They seem to have a minimal matriarchal society, as there is one "group leader" per pack.   
 
Deathclaws, originally thought to be mutated lizards, are mutant enemies in Fallout that are extremely strong and fast and have enormous claws on their hands.  Deathclaws encounters are more likely in areas of the Capital Wasteland furthest from settlements with the exception of the desolate town of Old Olney, where it is practically overrun with them. They can also be found in vast numbers in the underground Deathclaw Sanctuary.  Once a Deathclaw is killed, its corpse can be looted for a Deathclaw hand, which is needed to make the Deathclaw Gauntlet. 
Deathclaws are possibly the strongest enemy in some of the Fallout games (not forgetting Frank Horrigan), in Fallout 3 they are rarely second to trogs or even a powerful Yao Guai.
 
In Fallout 2 it is revealed that because of extensive testing using the F.E.V virus that Deathclaws intelligence could be drastically raised, causing them to be able to talk and use tools. You could obtain an intelligent Deathclaw companion, but after progressing through the game, he becomes less and less useful because of his inability to use ranged weapons. 
 
Deathclaws should never be taken on in close combat. Long range, such as sniping, is best to take them out and even being on/in an elevated position can protect you from most of its attacks. 
 
Fallout: New Vegas introduces several new types of Deathclaws, including Legendary, Blind, and Young/Child variants.