This pokemon used to live near Gringeycity with a lot of grimers. When Ash catches it, he could still smell the stench of Muk. When Professor Oak gets Muk, he couldn't stand the smell. We hear him at the end of the episode say, "Why did Ash have to bring me a Muk?"
Muk went against Janet's Bellsprout. No matter how many attacks that the Bellsprout used, it had no effect. Muk wins when he body slams the Bellsprout. Muk is currently at Professor Oak's lab.