Fog crawlers are large, mutated crustaceans which are the result of the radioactive fog altering the local shrimp population. The exact species from which fog crawlers mutated is not immediately apparent, but they are most likely a form of mutated mantis shrimp (specifically the Squilla mantis) based on the anatomy of their forelimbs and the presence of a distinctive neck which most crustaceans lack. Another potential candidate is the crayfish.
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 deep Fog for which they are named, and are seldom encountered outside the island's deadly mist. 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 and the ability to attack at range.
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 mid-four-digit 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.