Anglers are aggressive creatures native to the Island. They are usually found living in swamps or rivers where lure weed is present, sometimes hiding under the surface to ambush prey. The fleshy bioluminescent growth atop its head (similar to an angler fishes illicium) acts as a camouflage for the predator, effectively disguising it as the surrounding lure weed to fool its prey.
The canonical origins of the angler are unknown. They share elements associated with very different real world animals (namely fish and amphibians) that could be their theoretical ancestors. Whether the name Angler is intended to imply they are directly related to anglerfish or whether it's in reference to their glowing lure as a convergent feature is impossible to say without statement from Bethesda.
The angler is an aggressive creature with the ability to absorb the fog and spit it out like a fireball, inflicting incendiary damage to any target it is able to hit. In combat, they often rely on these long ranged attacks but have been observed to charge at their prey and use their claws, though often they will halt mid charge to use their preferred method.
With the means to use V.A.T.S., travelers can spot these creatures hiding among lure weed and avoid a surprise attack. Without this ability, the angler's illicium can be distinguished from the plant by the lack of leaves of a single appendage resting on the water's surface. Sometimes, the angler's head can be visible under the surface and mistaken for these leaves from afar but will remain submerged until the prey is close enough. Getting too close will alert the angler, causing it to spring from the water and engage.
Anglers' energy resistance is much higher than their damage resistance, making ballistic or melee weapons the preferable choice for engaging them in combat.