(From 5th Edition Monster Manual - 1994):

Of all the terrors created by foul sorcery, gibbering mouthers are among the most wicked and depraved. This creature is the composite eyes, mouths, and liquefied matter of its former victims. Tormented by the destruction of their bodies and absorption into the mouther, those victims gibber incoherently and are forced to consume everything in reach.

Amoeboid Form. The gibbering mouther’s body is an amorphous mass of mouths and eyes that propels itself by oozing forward, fastening several mouths to the ground and pulling its bulk behind. Though it moves slowly, it swims through water, mud, and quicksand with ease.

Horrifying Cacophony. When a gibbering mouther senses prey, its mouths begin to murmur and chatter, each with a different voice: deep or shrill, wailing or ululating, crying out in agony or ecstasy. This cacophonous gibbering overcomes the senses of any creature that hears it, causing most to flee in terror. Others are overcome with confusion or stand paralyzed, fixated on the horrific creature as it oozes forward to consume them.

All-Consuming. Driven to devour any creature it can reach, a gibbering mouther flows over victims transfixed by its ranting, its multitudinous voices temporarily silenced as it gnaws and swallows living flesh. The monster liquefies stone with which it comes into contact, hindering creatures that overcome its gibbering and attempt to flee.

A gibbering mouther leaves nothing of its prey behind. However, even as the last of a victim’s body is consumed, its eyes and mouth boil to the surface, ready to join the chorus of tormented gibbering that welcomes the monster’s next meal.

(From 3.5e Monster Manual - 2003):

This unwholesome creature has a body with the form and fluidity of an amoeba. Its surface has the color, but not the consistency, of human flesh. Countless eyes and toothy mouths constantly form and disappear all over the creature, often retreating into its body even as they become apparent. 

A gibbering mouther is a horrible creature seemingly drawn from a lunatic’s nightmares. Although not evil, it thirsts after bodily fluids and seems to prefer the blood of intelligent creatures. 

Sometimes the arrangement of eyes and mouths on a gibbering mouther’s body a resembles a face, but just as often they have no relationship to one another. 

A gibbering mouther is about 3 feet across and 3 to 4 feet high. It weighs about 200 pounds. 

Gibbering mouthers can speak Common, but seldom say anything other than gibberish. 

Combat: A gibbering mouther attacks by shooting out strings of protoplasmic flesh, each ending in one or more eyes and a mouth that bites at the enemy. A mouther can send out a total of six such members in any round. 

Gibbering (Su): As soon as a mouther spots something edible, it begins a constant gibbering as a free action. All creatures (other than mouthers) within a 60-foot spread must succeed on a DC 13 Will save or be affected as though by a confusion spell for 1d2 rounds. This is a sonic mind-affecting compulsion effect. A creature that successfully saves cannot be affected by the same gibbering mouther’s gibbering for 24 hours. The save DC is Charisma-based. 

Spittle (Ex): As a free action every round, a gibbering mouther fires a stream of spittle at one opponent within 30 feet. The mouther makes a ranged touch attack; if it hits, it deals 1d4 points of acid damage, and the target must succeed on a DC 18 Fortitude save or be blinded for 1d4 rounds. Eyeless creatures are immune to the blinding effect, but are still subject to the acid damage. The save DC is Constitution-based. 

Improved Grab (Ex): To use this ability, a gibbering mouther must hit with a bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. 

Blood Drain (Ex): On a successful grapple check after grabbing, that mouth attaches to the opponent. It automatically deals bite damage and drains blood, dealing 1 point of Constitution damage each round. A mouth can be ripped off (dealing 1 point of damage) with a DC 12 Strength check or severed by a successful sunder attempt (the mouth has 2 hit points). A severed mouth continues to bite and drain blood for 1d4 rounds after such an attack. A creature whose Constitution is reduced to 0 is killed. 

Engulf (Ex): A gibbering mouther can try to engulf a Medium or smaller opponent grabbed by three or more mouths. The opponent must succeed on a DC 14 Reflex save or fall and be engulfed. In the next round, the mouther makes twelve bite attacks instead of six (each with a +4 attack bonus). An engulfed creature cannot attack the mouther from within. The previously attached mouths are now free to attack others. The save DC is Strength-based and includes a +2 racial bonus. 

Ground Manipulation (Su): At will, as a standard action, a gibbering mouther can cause stone and earth in all adjacent squares to become a morass akin to quicksand. Softening earth, sand, or the like takes 1 round, while stone takes 2 rounds. Anyone other than the mouther in that area must take a move-equivalent action to avoid becoming mired (treat as being pinned). 

Amorphous (Ex): A gibbering mouther is not subject to critical hits. It cannot be flanked. 

Skills: Thanks to their multiple eyes, gibbering mouthers have a +4 racial bonus on Spot checks.

A gibbering mouther has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It always can choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.

(From Assassin Mountain: Monstrous Compendium Pages - 1993):

The gibbering mouther is an amoeboid form of life composed entirely of mouth and eyes. With its eyes and mouths closed, it appears to be a lump of earthly material, surprising creatures that stumble across it. Its only motive is to eat whatever is edible and within reach, be it animal, vegetable, or mineral.

Gibbering mouthers move by oozing forward, fastening several mouths to the ground and pulling themselves along. A mouther may move faster over fluid and viscous terrain, such as mud and quicksand, by swimming.

Combat: The brain of a mouther is located in its midportion, and its gelatinous body makes it difficult to strike this spot, hence its relatively low Armor class.

The mouther attacks in three ways: gibbering, spitting and biting. When any edible object is sighted by a mouther, it begins gibbering incoherently, causing confusion among all within a 60-foot radius who fails a saving throw vs. spell. Each character who fails must immediately roll 1d8 to determine which of the following effects occurs. On a roll of 1, the victim wanders aimlessly for one round; on a roll of 2-5, the victim stands motionless, stunned for one round; on a roll of 6-7, the victim attacks the nearest living creature for one round; and on a roll of 8, the victim runs in fear for two rounds.

The spittle of a gibbering mouther bursts into a bright flare if it strikes any hard surface. The resulting flash blinds characters looking at it if they fail to save vs. petrification — the blindness lasts 1d3 rounds. The mouther may then attempt to bite blinded opponents with a +2 bonus to its attack rolls. Blinded victims attack with a -4 penalty.

A mouther attacks by biting with six mouths per round. Each attack roll exceeding the number required to hit by 2 or more indicates that the mouth attaches to the victim and drains an additional point per round. When three or more mouths are attached to a single victim, that character must make a successful Dexterity check each round thereafter or slip and fall. The mouther will then flow over the victim and bite with 12 mouths, gaining a +4 bonus to strike its prone opponent. Once it pulls down one victim, a mouther tries to trap another.

If a victim reaches 0 hit points, he is absorbed into the mouther, giving it another mouth and pair of eyes, as well as 1 hit point permanently, up to the maximum for its Hit Dice. Only living flesh can be absorbed like this.

A mouther liquefies the ground and stone within a 5-foot radius and controls the consistency of the material, changing it to doughy quicksand. It requires 30 seconds to alter earth to quicksand, and a full round to mutate stone to earth.

Habitat/Society: Like other amoeboid life forms, gibbering mouthers reproduce by asexual fission. When a mouther has absorbed enough victims to gain maximum hit points, it splits in two. Each mouther has 4+3 Hit Dice (one has 17 hit points, the other has 18). The mouther retreats to some small, dark den before the four-hour process begins. When the two new mouthers recover from the dividing process (which takes 7+3d12 turns), each seeks its own new territory.

Gibbering mouthers avoid each other’s territories and even physical contact with one another. It’s believed that bringing two mouthers in physical contact forces them to merge, creating a larger creature with twice the size, HD, and number of attacks, but half the already-slow movement of the parent monsters. These great beasts strip the land so thoroughly that they generally die of starvation as soon as prey becomes scarce.

Ecology: Gibbering mouthers are unnatural creatures, usually created by foul sorcery and kept as guards by mages or obscene cults. Although they can survive in the wild, they are more scavenger than hunters, and they rarely establish reproducing populations in any but the most lush swamps.