Grimer to Muk is one of the lazier evolutions - a small pile of slime to a larger pile of slime? That's really about it - aside from its eyes looking a little more depressed (hey, you would be too if you were a giant pile of slime).

Still, odds are you wouldn't want to keep one of these things in your Pokeball. It's one of the few 'mons out there that have a pretty good likelihood of leaking.