(From Volo's Guide to Monsters - 2016):

A slime-covered worm of immense size, a neothelid is the result of the mind flayer reproductive cycle gone horribly wrong. On rare occasions, an illithid colony collapses, typically after an external assault, and the elder brain is killed. When that happens, the colony’s tadpoles are suddenly freed from their fate. They no longer serve as food, and in turn are no longer fed by their caretakers. Driven by hunger, they turn to devouring one another. Only one tadpole survives out of the thousands in the colony’s pool, and it emerges as a neothelid.

Abhorrent to Illithids. Among the strongest taboos in illithid society is the idea of allowing a mature tadpole to survive without implanting it into a donor brain. Under normal circumstances, any tadpole that grows larger than a few inches in length is killed by the elder brain to be food for it or for less mature tadpoles. Any tadpole that survives beyond that state is perceived as a threat to the colony, and the mind flayers organize hunting parties to exterminate the abomination. Lacking enough intelligence to be detected by an elder brain’s power to sense thoughts, neothelids warrant such precautions.

Savage Behemoth. As a feral thing, a neothelid knows nothing beyond the predatory existence it has lived so far and struggles to comprehend its new psionic abilities. Neothelids prowl subterranean passages in search of more brains to sate their constant hunger, growing ever more vicious. These creatures can spray tissue-dissolving enzymes from their tentacle ducts, reducing victims to a puddle of slime and leaving only the pulsing brain unharmed. They have no knowledge of their link to illithids, so they’re just as likely to prey on as on anything else.

(From 2nd Edition Monstrous Manual - 1993):

A slime-covered worm of dragonlike proportions, a neothelid leaves a poison trail of mucous in its wake. It brings its prey down with flesh-corroding exhalations that dissolve all but the fatty brain, an essential component of its diet. An adult reaches 10 feet in diameter, but its wormlike body grows longer throughout its existence — reaching lengths in excess of 120 feet.

Four 20-foot-long tentacles sprout from an adult’s eyeless “face”, thrashing around a circular maw large enough to swallow man-sized prey whole. Dangerous psionic talents round out what may be the underdark’s most fearsome predator.

Its mauve coloration has caused some to mistake neothelids for purple worms at a distance; thankfully, both creatures are rare.

A neothelid senses its world via innate telepathy with a radius of 360 feet. Within this radius, the neothelid is aware of all non-psionically shielded sentient beings and can communicate with intelligent creatures via telepathy — even through solid rock.

Combat: When a neothelid corners its prey, it first resorts to its flesh-dissolving breath weapon. This weapon is not an acid, but rather an enzyme that liquefies most organic tissue. A neothelid can use its breath weapon twice per day, affecting a cone-shaped area 5 feet wide at its origin, 60 feet in length, and 20 feet wide at its extreme edge. Living creatures caught in the blast take 12d12+6 points of damage (successful saving throws vs. breath weapon reduce damage by half) as their flesh sloughs away. Any creature killed by the breath weapon completely melts, save for its clothing, possessions, and exposed brain.

Any prey that survives a breath weapon attack must face the neothelid’s impressive physical arsenal. An attacking neuthelid coils and strikes with its gargantuan body from sufficiently large subterranean caverns — much like a surface python. If its attack is successful, the creature bites for 6d6 points of damage or flails with its four 20-foot-long tentacles for 3d6 points of damage each. If a neothelid needs to, it will brings its psionic abilities to bear on its victims.

A neothelid’s body glistens with poisonous mucous; a slime trail marks its passage. Those who come in contact with the mucous murt make a successful saving throw vs. poison or face death in 1d10 turns (mind flayers are immune to this poison).

Habitat/Society: In the remarkably rare event that mind flayer tadpoles develop in the absence of illithid adults, a small number survive long enough on their own to develop into neothelids. These creatures indiscriminately hunt any illithids they may chance upon. Illithids, in turn, respond to the news of neothelid emergence with utter disgust, and they quickly organize hunting parties to exterminate the abominations.

Ecology: An illithid tadpole that fails to undergo ceremorphosis gradually grows into a neothelid — assuming it survives its first 100 years as a mindless slug eking out a living on subterranean vermin. Once a tadpole is powerful enough to be called a neothelid (triggered by the eating of its first sentient brain), it becomes a terrible danger, hunting all other living creatures (including illithids) wih an insatiable, insane hunger.

Unlike some other large predators of the underdark neothelids an unable to burrow through solid rock, forcing them to hunt naturally or previously delved tunnels and caverns. When a local subterrineai ecosystem is exhausted, neothelids may move to “greener” tunnels via their psychoportive abilities.

The existence of the neothelids engenders a host of unanswered questions regarding the actual nature and true origin of illithids — a mystery even to the illithids themselves.

(From Planescape: Morte's Planar Parade - 2023):

Originally created by the mind flayer god-brain Ilsensine and now produced by some of that god’s followers, eaters of knowledge are lumbering, bipedal masses of squelching muscles and exposed brain matter. These rugose hulks collect information from others by devouring brains before returning to their masters with delicious secrets. Unlike illithids, which overwhelm their foes with psionic power, eaters of knowledge use their physical strength to hold prey while burly feeding tentacles crack free their victims’ brains. Consuming brains fuels these brutes’ psionic power, making eaters of knowledge deadlier with each brain devoured.