Fog crawlers are large, mutated crustaceans, which were created as a result of the explosion of the Vault 94G.E.C.K.. This explosion coated the area that would become The Mire with a highly radioactive fog, and rapidly altered the local plant and animal life. This fog emulated the conditions found centuries later on The Island, allowing many similar mutations to take place in the local Appalachian wildlife. The exact species from which fog crawlers mutated is not immediately apparent, though the most likely culprit is the crayfish, a creature which shares much of the same biology as fog crawlers, and can commonly be found throughout swamps and marshes in North America.
The different types of tail "fans" on different types of fog crawlers could determine their gender, as crayfish carry eggs under the tails or as courtship during mating to avoid injury. The smaller legs (swimmerets) on the underside of the fog crawler are a holdover from when it remained an aquatic creature, assuming it isn't still largely amphibious. It seems that at some point the creature's head had evolved a mobile neck similar to terrestrial insects, as well as morphed into a kind of arrowhead shape. They seem to have eight eyes, four in a straight line on each side of the head, with the one second from the front being the largest. They also seem to lack any sort of visible mouth.
They live in the deepest, murkiest swamps of The Mire, and are seldom encountered outside of the region. Fog crawlers are known to make deep, booming vocalizations as an indicator of their presence. They are also among the strongest and most tenacious foes encountered in their respective areas, combining excellent protection with a generous health pool.
The fog crawler's main attacks are slicing with its arms and jabbing with its head. It also has an attack where it jumps up and slams back into the ground, dealing massive amounts of damage to anything caught in the area of effect. This ground slam can deal damage even to targets well above the crawler's height level, such as human prey on top of some object the crawler cannot mount. Due to their erect prosoma, fog crawlers can also strike at elevated targets with their arms.
Unlike mirelurk queens or super mutant behemoths, fog crawlers lack any form of ranged attack, a shortcoming they compensate for with deceptively high speed and agility. The monstrous creatures can cover considerable distances very quickly, thus making it hard to shoot them down before they enter melee range. Their speed also makes it more difficult to escape from them. Fog crawlers have characteristic sounds they emit, allowing one to detect when they are in the area.
As mentioned above, especially the fog crawlers' higher-level variants are extremely heavily armored, possibly boasting resistances in the 100-125 range coupled with a large health pool. They are easily the most resilient threat the player character can come across in Appalachia. Energy weapons are the creatures' only mentionable weakness due to their slightly lower Energy Resistance. Due to their armor-reinforced carapaces providing them with high damage resistance, ballistic weapons modified with armor-piercing upgrades can pack a great deal of damage against them. If their legs are crippled, they will move significantly slower.
A fog crawler's ground pound attack will ignore any block attempt and stun the player character for enough time to deal additional damage with its fast slicing attacks.