A Deathclaw is an overgrown, mutated form of Reptile. It's believed that they were Jackson's Chameleons before they were modified with the DNA of other species. They were created before the war by the United States military in order to take the place of humans during the most dangerous missions.
In Fallout 3, Deathclaws were one of the most powerful and durable species in the game before the arrival of the DLC pack, Broken Steel (which including Feral Ghoul Reavers, Super Mutant Overlords and Albino Radscorpions). They move extremely quickly and their swipe attack has the power to knock you back a couple of feet. Luckily, Deathclaws usually travel on their own and the majority of them don't appear until later in the game (although many people have spotted Deathclaw couples after adding Broken Steel).
There's two locations in the Capital Wasteland that are crawling with numerous Deathclaws, they are Old Olney and the Deathclaw Sanctuary, there's also certain Deathclaws that will spawn no matter what level you are, such as the one that appears at the F. Scott Key Trail & Campground and the random encounter with a Wastelander fighting one.
Deathclaws can be easily slowed down and killed with the Dart Gun, this will slow down the Deathclaws making them an easy target.
 Fallout: New Vegas
Related ThreadsThe Legendary Deathclaw Help (Possible Spoiler) - last post by @ Mar 15, 2011
Alpha Male and Mother Deathclaws - last post @ Feb 5, 2015
The legendary Deathclaw (very hard one shot) - last post by @ Nov 9, 2010
Deathclaw confusion... [Spoilers] - last post @ May 14, 2012
Are all deathclaws in the wasteland have killing me as their objective? - last post by @ Dec 26, 2009