The Gibbering Mouther is an aberration from the Dungeons & Dragons game, and is essentially a royalties-free expy of a Shoggoth. This repulsive slime-like creature appears as a shapeless blob adorned with a constantly shifting array of eyes and teeth, which at least some editions claim are the remnants of victims it has engulfed and absorbed into its repulsive slimy bulk. Getting around by extending pseudopods covered in maws that bite into the earth so it can drag itself forward, its name stems from the fact that its countless mouths constantly shout, whisper, scream, plead, chant, rant and generally spout utter gibberish. This makes them very disturbing creatures to fight and they are surprisingly nasty.
The Epic Level Handbook introduced the Gibbering Orb, a powerful monster with a challenge rating of 27 that may be the common ancestor of both the gibbering mouther and Beholder, as it not only has the gibbering ability of the mouther, but also has 24 different eye rays, plus it can bite with 12 mouths, and when it eats somebody can steal their spells. And they can speak any language and are very intelligent despite their gibbering making them seem insane.
4th edition gave these suckers a surprising new twist by creating a whole family of "Gibbering Beasts". The 4e Gibbering Mouther was just the weakest of them all, as a level 10 Controller, but it retained its own particular flavor of nastiness. It had the Warped Ground aura, reflecting its traditional ability to make pseudo-rough terrain out of its environs due to a combination of constant munch and soaking everything in acidic drool, could swim, had a bite attack that inflicted ongoing acid damage, could daze victims with its gibbering, and had a special "Gibbering Feast" attack, where it spewed gobbets of itself over everyone nearby, which caused little mouths to appear on their flesh and start trying to eat them alive. Its big brother (level 18 Controller) was the Gibbering Abomination, a flying tentacle monster of a Gibbering Beast that slapped its victims with brain-frying tentacles and could immobilize them with eyebeams in addition to gibbering like the Mouther. In fact, it was even nastier to listen to, with its Unnatural Utterances aura imposing a permanent attack roll penalty on anybody who got too close. Nastiest of them all, however, was the Gibbering Orb; this level 27 (for reference, 4e characters go up to level 30) Solo Controller was essentially the unholy bastard offspring of a Gibbering Mouther and a Beholder. It had a horrific array of half a dozen eye-beam attacks (Mindcarving Ray, Flesheating Ray, Bonewarping Ray, Bloodfeasting Ray, Farsending Ray, Souleating Ray), a bite attack that would detach a mouth to continue eating its victim, and a Merciless Eyes aura that let it get off free bonus eye-beam attacks on anyone who was too close. Plus the bog-standard gibbering effect.