MUK's flesh is literally a layer of thick sludge. If it doesn't eat enough poison gas, the sludge hardens and this POKeMON becomes completely immobile.
MUK is made when a GRIMER falls into a vat of radioactive toxic waste. This waste causes it to mutate, and become more poisonous. Standing around one too long can make one sick.
MUK's body releases poison gases that are harmful to humans and other POKEMON. If it comes near polluted waters, it grows bigger by absorbing the water and turning it into sludge for its body.
(Probably the worst idea I've had thus far, but it's worth a shot.)