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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 grey 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.
It 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.