Muk

The final stage of evolution for Grimer, Muk is thickly covered with a filthy, vile sludge and it is so toxic even its footprints contain poison. While fighting off Magnemite, Magneton and a whole mess of Grimers in "Sparks Fly for Magnemite," Ash engages in a heart-stopping Pokemon battle with an enormous Muk. After Ash unclogs the stopped-up water pipes in the power plant, he catches Muk for his Pokemon collection!

First appearance: Episode 30—"Sparks Fly for Magnemite"