You can smell it from a mile away.
A big, horrific pile of glop. Perhaps it's made of toxic sludge from a Corrupt Corporate Executive's industrial plant, or perhaps it's merely a colossal pile of dung.
But it's alive.
The Muck Monster is a living embodiment of filth and slop, and the Green Aesop tends to be his stomping grounds. Nothing tends to hammer an anti-pollution message home like an colossal, animate pile of (sometimes literal) crap. Plants die from this beastie's approach; sometimes animals and even people bite the dust from being in mere proximity to this abomination.
Compare Blob Monster, which is more transparent and tends to keep itself in one defined and consistent mass rather than constantly oozing and leaving parts of itself in its wake, (and generally less smelly), and Talking Poo. See also Toxic Waste Can Do Anything, where the toxin creates a monster without being sentient itself.
- The Zingy animated icon used by British energy company EDF has often been likened, unfavourably, to a parasitical turd-monster. Embittered website Ad Turds has not been slow in likening it to South Park's Mr Hankey in its relentlessly cheerful disposition while - well, remaining a turd. The advertising agency has responded by making its colour a more orange-yellow rather than brown: but the current advertising depicting a very-large Zingy in the form of a hot-air balloon does make it look like a giant turd floating above British suburbia.
- Ginta uses Babbo: Alice to dispel this curse off "Gramps" in episode 97 of MÄR.
- Spirited Away has the Stink God, who is literally just a moving pile of sludge with eyes. His smell is so bad that it makes everyone's eyes water and makes all of the food around him rot. He turns out to actually be a River Spirit, and his filthy state was due to pollution in the river he inhabits.
- Batman's enemy Clayface (all eight of them) fit the Trope in some way, although some are more humanoid than others. The worst part is, all of them were once human, except for the fifth one, who is a child of Clayface III and Clayface IV born AFTER they each gained their powers.
- Sludge from The Ultraverse. Frank Hoag was a Dirty Cop who was killed and covered in chemicals by an explosion before his body was dumped in a sewer. The chemicals had regenerative properties and tried to heal Hoag, but combined the sewer substances with his body, transforming him into a huge mass of living slime.
- Tara Infirma, one of the minor characters in All-Ghouls School.
- FernGully: The Last Rainforest: Hexxus, the Tim Curry-voiced pollution-demon villain, although he was the primordial spirit of destruction before he latched on to human inventions. His forms varied from a sludge-based blob monster to an exhaust fume-based ghost to a giant burning skeleton made of tar.
- Creepshow 2 had a large, flat monster resembling an oil slick, that dissolved any flesh it touched, inhabiting a lake.
- That was an adaptation of The Raft, a short story by Stephen King.
- Godzilla vs. The Smog Monster, villain Hedorah has to be the most famous example of this trope, and it shows why one must never let alien spores come into contact with industrial waste, lest it mutate into a giant tadpole sludge demon that can drop lethal acid as it flies.
- The H-Man is an earlier iteration of this, but explicitly radioactive (hence the name). It was once a man, and now must eat other humans in order to retain its shape.
- Dogma had a demon, the Golgothum, constructed from crap - specifically, the crap of all the prisoners crucified at Golgotha (yes, including Him). He's defeated by deodorant (which "knocks out strong odors").
- Armageddon from The Return of Hanuman is a monster formed from a volcano, mostly because of the many inorganic trash contained inside and activated when Rahu and Ketu's staff went inside of the volcano.
- MonSturd. It's like Jack Frost (1997), except the Serial Killer was genetically fused with shit instead of snow.
- The low-budget 1986 horror film Spookies had Muck Men, who made explosive flatulence sounds when they came to get you (which presumably caused a bit of confusion among viewers as to whether they were muck monsters or poo monsters).
- Visser Three from Animorphs is a shapeshifter with two forms like this, seen in The Weakness and The Hidden respectively.
- During the lifeforce crisis in Reaper Man, one of the Unseen University's compost heaps comes to life. It attacks the gardener and dissolves its way through the main door before the wizards blew it up with a bottle of Wow-Wow sauce.
- The deities Abhoth and Ubbo-Sathla in Clark Ashton Smith's Cthulhu Mythos stories.
- The title creature in Slime by William Essex: a living lake of toxic waste that eats people and animals.
- In The Royal Book of Oz, the first Oz story by Ruth Plumly Thompson, the Scarecrow briefly encounters Middlings, man-like creatures made of mud.
- Eva Mudlark of The Aquabats! Super Show! has made several of these out of anger from being treated as a lowly garbage collector.
- Doctor Who: In "Gridlock", the Macra live at the bottom of the Motorway and feed on the gas fumes created by the huge numbers of Flying Cars stuck in the universe's worst traffic jam.
- Call of Cthulhu had a version of Clark Ashton Smith's Abhoth (see above).
- Dungeons & Dragons had a number of these, such as the grey ooze and the demon prince Juiblex.
- The nuckalavee from the Basic/Expert/etc D&D system lives in fouled water and kills small animals by its very proximity. A subversion in that it's not a product of pollution or a Green Aesop, and it looks like a skinned centaur with a Fish People head rather than a pile of trash. Based on a monster from Scottish legend.
- A swamp that's tainted by magical pollution can create mudmen, which are Exactly What It Says on the Tin. Not very intelligent, they try to drown anything that enters their swamp out of an instinctual urge, often destroying themselves in kamikaze attacks as they try to smother their foes.
- The Ravenloft Darklord Malus Scleris rules a domain that is the horror of unlimited pollution made real, and it is filled with these kind of monsters. Scleris is a pre-industrial version of a Corrupt Corporate Executive; his Freudian Excuse is that his father, a druid, only cared about his work, not about his son, so in return he hates Nature for taking his father away from him. His MO is control of horrible and disgusting diseases.
- Moander, a minor evil god from the Forgotten Realms, manifested in the form of a gargantuan crawling heap of rotting vegetation.
- In Shadowrun, city spirits usually manifest as piles of ordinary litter, unsightly but not all that icky. Toxic nature spirits manifest as contaminated water, smog, or really foul rubbish.
- Pokémon Red and Blue has two Muck Monsters in Grimer and its evolved form, Muk. In the Pokémon anime, Ash's Muk was (initially) so smelly that it stank even after it had been captured in a Poké Ball. When he teleported it to Professor Oak's, it proceeded to engulf half the lab and reduced Professor Oak to ranting.
- Of course, said Muk really just wants a hug.
- Koffing and Weezing count as this as well. They're based on pollution, but they're more inspired by smog that slime, although they do possess the ability to spit globs of slime.
- Pokémon Black and White introduced the garbage Pokémon Trubbish and Garbodor. Their bodies pick up trash as they go, so they're ironically useful in keeping places clean.
- Master Belch and his upgraded form, Master Barf, from EarthBound. There are also the smaller monsters like "Pile of Puke" and "Stink Spirit".
- The Slag Golems enemy group in City of Villains.
- Digimon has Raremon, a crawling, blubbery pile of rotten flesh entangled with random cybernetics. It smells like dead fish and vomits as an attack. Raremon is one of several "garbage" type Digimon, monster who basically failed their evolution. Others include Sukamon, a living pile of feces, the slug-like Numemon, and Garbagemon, which even lives in a trash can.
- Wild Arms 2 had the boss "Drawdo," which resembled several dead fish melded together and attacked with every status effect in the game: https://www.youtube.com/watch?v=7gapGzA_fjw
- Demon's Crest on the SNES has the boss "Crawler," a huge mass of melted-looking flesh that takes on a semi-humanoid form after it swallows some bones. It can spawn zombie-like creatures from its body.
- Dragon's Lair has mudmen, which unlike the other monsters cannot be killed, only avoided.
- The Smoky Progg from Pikmin could count too as it is a horrifying crawling cloud of polllution which instantly kills any Pikmin it touches. Not even unpicked ones are safe from its wrath.
- One of the bosses from Conker's Bad Fur Day was the Great Mighty Poo.
- Warcraft III has some sludge-type creeps.
- The Amoeboids in the Ratchet & Clank series are a kind of slime monster that walks in circles and says "Nein!". When hit, they split into two, half as small slime monsters (except for the smallest sort, which just disappears). The biggest ones will split five times, meaning that you have to kill 63 slimes in total.
- Bulb Boy has a Puzzle Boss based on this concept. After the protagonist eats the cooked corpse of an evil zombie chicken the resulting bowel movement creates a large worm-like turd monster that must be flushed down the toilet before it eats Bulb Boy.
- The Mud Bog areas in the Shantae series are filled with Mud Bog creatures made out of the stinky, sludgy mess that the area is filled with. They either jump around in a blobby mass or pop out of the swampy floors and ceilings.
- The Binding of Isaac has a lot of poo-themed enemies, including Dips, Squirts and Dingas (each progressively larger and which split into their smaller kin when killed), and bosses Dingle, Dangle, the Turdlings and Brownie.
- Crud in The New Adventures of Winnie the Pooh episode "Cleanliness Is Next To Impossible", a creature made from the filth under Christopher Robin's bed. He's defeated with a bar of soap.
- One episode of Goof Troop featured a Corrupt Corporate Executive who had become a Muck Monster due to overexposure to all the pollution created by his factory. Goofy returns him back to normal with a jar full of fresh air.
- On several occasions the mutagen on Teenage Mutant Ninja Turtles spawned these creatures; they actually had names like "mud monsters" and "Mutagen Man." Basically, the theory that that this is what happens when the mutagen produces something that doesn't have the dignity of resembling a particular animal like a turtle. (And of course, the accidental workings of a mutatagen in Real Life are much more likely to produce a monstrosity like this than an anthropomorphic turtle....)
- In the ThunderCats (1985) episode Mandora: The Evil Chaser, one of villains is a Muck Monster known as The Living Ooze. Defeated with a powerful weapon of the ancients - SOAP!
- The Tick animated series featured "The Filth," slimy sewer-dwelling creatures with, erm, tapered heads.
- And a variation with a snot-based clone of the Tick himself created by the being from the alternate dimension down the hall. Not exactly a blob of muck, but it was made of snot/mucus and, while shaped like the tick, could squish itself into other shapes and use it's semi-gelatinous consistency to seep through cracks and around attacks and such.
- In Lilo & Stitch: The Series, there's experiment 505, Ploot, who is designed to collect and absorb pollution and turn it into a toxic sludge to flood cities with. The more he collects, the bigger and grimier he gets.
- They were actually made of plain old mud, but in several episodes of the Aladdin: The Series regular series, the heroes ran afoul of the Al-Muddies, who were just as dangerous as any Muck Monster. These elemental spirits were made of living mud, lived underground, and were carnivores known to prey on humans. Normal ones were feral, dimwitted, and brutish creatures who tended to attack in mobs; their Sultan, however, stands out, not just because of his titanic size, but via his incredible cunning and intelligence (not to mention being able to talk), and a fondness for cooking victims. (Even going so far as to call himself "a gourmet".)
- Mudman from Atomic Puppet, a minor member of the Rogues Gallery able to possess other beings, shapeshift, and regenerate From a Single Cell.
- In Samurai Jack, a group of mud people come to offer Aku a tribute for being allowed to live on Earth (An equally muddy statue of Aku). All Aku really cares about is that they are dripping mud all over his floor.
Aku: I just had it vacuumed! Out! Out!
- Naturally Captain Planet had an episode with one of these. In this case it was an artificial organism that was supposed to eat garbage, but of course when Sly Sludge used it, it went horribly wrong and turned into a Grey Goo that threatened to turn the entire planet into one big pile of living garbage.
- Most of the foul-smelling slime found in stagnant ponds, along the tide line of beaches, and other messy natural settings is made up of mats of living bacteria by the quadrillions. They don't walk en masse, but individual bacteria do move around within their filmy habitats.