Muk is a large purple mass of sludge, toxic fluids, and other filthy materials. It has two large arms that capture prey and its eyes are barely visible through the sludge.
Muk can have the ability Stench or the ability Sticky Hold. Stench decreases the chances of encountering a wild Pokémon when Muk is the first party Pokémon. Sticky Hold prevents the theft of Muk's held item. A toxic fluid seeps from its body, which can instantly kill plants or trees on contact. Muk has one of the highest attack points of all Poison-type Pokémon, losing only to Toxicroak.
In the anime, Ash has a Muk after he caught it to restore the power of a town. He later used it to battle a Bellsprout used by a trainer called Jeanette and was able to beat it thanks to Muk being able to absorb all of Bellsprout's attacks, finally finishing it with a Body Slam.
Muk has the shortest name of all Pokémon, tied with Mew.
Muk, Garbodor and their prevolutions are based on imagined fears that heavy pollution, caused by the leakage and improper disposal of radioactive waste, may cause, the creation of new and undesirable lives.
Muk, like Grimer, has a horrible stench that its sludge body produces. However, in all of the Oak's Lab scenes since it's capture, Ash's Muk supposedly doesn't have this trademark smell after it was sent to the lab.