Chuuls are a species of Aberration from the worlds of Dungeons & Dragons that vaguely resemble a giant lobster with a mouth surrounded by tentacles that secrete a paralytic slime, similar to a Carrion Crawler. Uniquely amongst big-name aberrant critters, Chuuls were invented for Dungeons & Dragons 3rd Edition, having debuted in both the 3.0 and the 3.5 Monster Manual; they've appeared in the Monster Manual for both editions that followed, which is a pretty impressive feat!
As their appearance suggests, chuuls are amphibious creatures; roughly humanoid in terms of intelligence, but antisocial. They dislike interacting with most humanoids as anything other than food... fortunately, they're very well equipped to make a meal out of any trespassers. However, they're not particularly great swimmers; whilst they lair underwater, they hunt along the shoreline, attacking prey either on land or in the shallows. Their typical hunting strategy is to wholly or partially submerge themselves in murky water off the shoreline and wait for victims to get within range of a charge attack; once brought to melee, they attempt to grab a victim and drag it into range of their tentacles, allowing them to paralyze a victim and begin crushing/gnawing it to death. They keep one claw free to try and fend off attackers as they feed, but are smart enough to drop a paralyzed victim to then attack other major threats.
Chuul lairs can be found in swamps, jungle lakes, temperate marshes, underground rivers, coastline shipwrecks and even sufficiently moist sewer tunnels. Older chuul in particular will dig up lake bottoms and use trees and stones to craft rudimentary structures to hide in. Open water, in the form of rivers and oceans, are mainly used to transport themselves to new locals. Chuul hunters can expect to face chuuls as either loners, pairs, or in pods of 3-5 individuals. A chuul lair consists of roughly circular rooms that are interconnected by tunnels made just large enough for the biggest member of the resident pod to squeeze through. One of the aforementioned rooms will always be a trophy room, as chuuls are born hoarders, and always keep aside trophies taken from humanoid victims; shiny armor, a piece of jewelry, even a well-crafted saddle. If nothing else, a carefully emptied skull will be harvested - chuuls find humanoid brains to be poisonous, which gives the race a strong propensity to work with illithids. They often work their trophies into macabre art, and can even be persuaded to barter with the items in their hoard, but only for captive humanoids or other things a chuul considers a "delicacy".
Hunters had best beware, because, as mentioned above, chuuls are at least as smart as people, but with no ambitions beyond a primitive lifestyle of hunting and sleeping. This is aided by a powerful racial memory, which is the source of their deep, unquestioning hatred for all surface-dwelling intelligent humanoids - only other aberrations or humanoids of the Underdark are exempt from this
The Uchuulon, or "Slime Chuul", was introduced in the Stormwrack splatbook for 3e. Created by implanting a chuul with an illithid tadpole, the result is a sluggish, weak-shelled chuul that constantly secretes a thick layer of paralytic mucus from its entire body.
Chuuls have their own "Ecology of the Chuul" article in Dragon Magazine #330.