Muk is a large, sticky, amorphous Pokémon made of living purple sludge. It has two small eyes with beady black pupils. It also has an enormous mouth with a gray tongue and strands of ooze connecting its top and bottom jaws. While it has no visible legs, it does have two arms with three fingers on each hand. However, it typically keeps one hand tucked in while it extends the other hand.
Muk is a living biohazard, leaking toxins that instantly kill all plant life it touches. Even it being in close proximity to plants will cause them to wilt and die. Extreme caution is advised in avoiding any contact with this Pokémon, as an accidental brush up can lead to severe sickness. The effects of this Pokémon can leave the landscape barren for three years. In dirty residential areas where people throw trash into the streets, it is almost certain to appear. It will readily consume nearly any waste or refuse generated by people and Pokémon alike. It can be found in heavily polluted bodies of water, cities and factories. However, Muk's population has declined due to recent environmental improvements and extermination efforts. This has led some to believe that it will go extinct at some point; sludge ponds are being built to prevent its extinction.
Alolan Muk has a more vibrant coloration, consisting of green, pink, and yellow stripes with thin blue bands in-between. The green, yellow, and pink stripes are capable of shifting downwards while the blue stays in place. It has various white, crystallized poison shards protruding from parts of its body, especially from its mouth and fingers, giving the impression of claws and fangs. It has blue eyes with a black space behind them and a bright blue tongue. Its vivid coloration is a result of the garbage it eats, which causes chemical changes in its body. Its body contains over a hundred different kinds of poisons.
Alolan Muk's unexpectedly quiet and friendly. However, if it is not fed trash for a while, its hunger will compel it to destroy its Trainer's furnishings for food. It is thought that it cannot maintain its energy with a constant source of toxins, which causes its voracious appetite. The crystals covering its body have a tendency to break off, which makes it extremely difficult to deal with due to the concentrated toxins contained within them. This variant of Muk does not produce the foul stench due to its toxins being contained in with its body. Alolan Muk preys on Trubbish and Garbodor.
Muk debuts in You know... Articuno! where it was used in by Harry to help Team Rocket on their mission to get Articuno. When Red tried to stop them, Harry sent Muk against Red's Gyara. It was eventually frozen by Articuno.
Muk is one of the Pokémon owned by Koga in Karate Machop!, where it was used on Cerise Island to battle the Elite Four, and proved useful in Koga's survival after the island began to crumble. In the HeartGold & SoulSilver chapter, it was used during the Circle Push event of the Pokéathlon.
Although Muk's body is very poisonous, it doesn't seem to harm Ash and the others when Muk is 'hugging' them. This is probably because it is able to control whether it gives off poison or not, similar to how Ponyta can choose whether or not to burn the person that touches it. Croagunk is another Pokémon which is able to control the amount of poison it uses via its fingertips when using Poison Jab, or Sharpedo activating its Ability, Rough Skin.
Muk shares similar traits to Weezing and Garbodor. All three Pokémon are pure Poison types with a single pre-evolution, roughly the same base stat totals and evolution levels, and appear to be based on a different kind of pollution; Muk being water pollution.
Alolan Muk and its Alolan pre-evolution were designed by Hitoshi Ariga.
Muk is one of two Pokémon to have the shortest English Pokémon name, Mew being the other.
Muk appears to be a pile of living sludge. It may also be based on the imagined fear that heavy pollution, caused by the leakage and/or improper disposal of toxic/radioactive waste, may result in the creation of new and undesirable life.
Alolan Muk may be based on an oil slick, particularly the rainbow colors it produces when it mixes with water. In the real world, oil spills are a major threat to the ecosystem of the world's oceans (a particular danger in Hawaii, which Alola is based on).
Muk is a corruption of muck (generally something filthy).
Betbeton may be derived from べとべと betobeto (sticky) and ton.