Muck Monster

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, the Green Aesop tends to be his stomping grounds. Nothing tends to hammer an anti-pollution message home like an colossal, animate pile of 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 generally less smelly), Talking Poo.

Examples of Muck Monster include:

Anime and Manga

Ginta uses Babbo: Alice to dispel this curse off "Gramps" in ep 97 of Marchen Awakens Romance.
In Pokémon, the aptly named 'Mon Grimer and its evolution Muk are made of toxic slime.
Spirited Away has the River Spirit that was mistaken for a 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. It's due to the pollution in he river he inhabits.

Film

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.
- Hedorah comes with a Meaningful Name with a dash of Exactly What It Says on the Tin as the name is derived from hedoro which is Japanese for "Sludge of Industrial Waste".
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.

Literature

During the lifeforce crisis in Reaper Man, one of the Unseen University's compost heaps comes to life. It attacked the gardener and dissolved 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.

Tabletop RPG

Call of Cthulhu (tabletop game) had a version of Clark Ashton Smith's Abhoth (see above).
Dungeons & Dragons had a number of these, such as the Shambling Mound 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.
- 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.

Video Games

Pokémon has two Muck Monsters in Grimer and its evolved form, Muk. In the 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 Wheezing arguably 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.
- Black and White introduced the garbage Pokemon Trubbish and Garbodor.
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: http://www.youtube.com/watch?v=7gapGzA_fjw
Demons 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.
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.
A recurring enemy in the Ratchet and Clank series is 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.

Web Comics

The Non-Adventures of Wonderella has A variation.

Web Original

One of the monsters in Monster Basement actually is called "Mud Monster". It makes a brief reappearance in Monster Basement 2.

Western Animation

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.
Hexxus, the Tim Curry-voiced pollution-demon Big Bad of Fern Gully.
On several occasions the mutagen on Teenage Mutant Ninja Turtles spawned these creatures; they actually had names like "mud monsters" and "Mutagen Man." Bascially, 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 episode Mandora: The Evil Chaser, one of Mandora's criminal prisoners is a Muck Monster known as The Living Ooze.
- Defeated with a powerful weapon of the ancients - SOAP!
The Tick (animation) 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 (34B, IIRC?). 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.