Deathclaws are extremely dangerous creatures found in the Commonwealth, The Island and in Nuka-World.
Originally created before the Great War by the government to replace human troops in battle, deathclaws were derived from mixed animal stock, primarily from the very common Jackson Chameleon. They were then refined by the Master, using genetic manipulation and the Forced Evolutionary Virus. The resulting creature is incredibly fast and powerful.
Around 2161, deathclaws were still considered mysterious and legendary creatures to many. They could originally only be found in the Boneyard at the warehouse, living with their mother. Later, as the deathclaw population grew and began to spread across the continent, the deathclaws stopped being perceived as legendary beasts, and started to be seen just as another dangerous creature.
 
Characteristics

Deathclaws are tough and formidable melee combatants. Their appearance is also more detailed compared to their Fallout 3 and New Vegas predecessors; they have darker, scaled skin, longer horns (which seem to curve more downward rather than upward), shorter claws and a more draconic/reptilian head and face. Close inspection of their feet will reveal what appears to be a single enlarged toe claw on each foot, similar to prehistoric raptors. The Massachusetts deathclaws are more physically dynamic; showing abilities such as running on all fours when in combat or climbing obstacles, behaviors never seen in the deathclaws of previous titles. Deathclaws can be found anywhere in the Commonwealth, but appear more frequently in the Glowing Sea. When observed from a distance, deathclaws can be seen to throw dirt over what could possibly be egg clutches. They're also known to perch atop hills, rock outcroppings, and boulders surveying the landscape.

Biology

Deathclaws are the results of a pre-War experiment that involved creating a mutation of the Jackson's chameleon, along with several other species, to use as weapons on the battlefield. They are strong, agile predators that will attack the player character on sight. Their long, sharp claws are capable of inflicting great, oftentimes deadly amounts of damage (hence their name).

Gameplay Attributes

Deathclaws have a very high amount of health for creatures of their level and are incredibly resistant to most damage types, particularly energy damage. Their versatile combat style makes them a hefty opponent even if they are lower level. When aggravated, they will initiate combat by roaring and stomping the ground, causing a shockwave.
While they predominantly use their namesake claws for dealing damage, they may also headbutt their foes to stagger them. Their power attack comes in the form of a wide-swinging claw slash which deals significant damage. If one is available, they have also been noted to use both hands to throw cars at an opponent who is out of melee distance.
Deathclaws have the ability to instantly kill the Sole Survivor or other humanoids, either by grabbing and smashing them into the ground, or decapitation with its claws. This maneuver only occurs when the Sole Survivor has low enough health, although this is relative - higher percentages of health at low level can still result in the instant-kill attack, but at higher levels it may not occur until the player has very low health. Other times, the deathclaw may pick up the player and slam them into the ground forcibly, causing moderate damage. The instant kill can also be nullified by wearing any type of power armor. Instead, the deathclaw will grab the Sole Survivor and stagger them, draining their AP completely. A deathclaw will also toss sand into the air to provide a brief distraction.
The deathclaws have a substantial level of natural armor and are resistant to damage on their dark, scaled parts, leaving the belly as a weak spot which takes increased damage. However, due to their hunched posture, this is difficult to hit. Although the Penetrator perk allows the Sole Survivor to hit the deathclaw anywhere while using the Vault-Tec Assisted Targeting System, this works by shooting through the deathclaw like it is a solid object. They can dodge gunshots and zigzag while charging the shooter, making them even harder to hit. If one of their legs become crippled, broken bone will be seen stabbing out of the skin, and they will move incredibly slowly. With both legs crippled it is unable to move at all. It is also possible to shoot off the end of their tail.
Their model size makes them ungainly in uneven ground, which can be used against them. Their inability to climb or jump (aside from scripted events such as in Concord) renders their close-combat style useless if attacked from high enough ground, and if they are unable to travel certain terrain such as jumping up on a rock, they will flee until their target is on level ground again. Many of their attacks can also be avoided by keeping a constant pace, which allows the player to simply outrun their claw strikes