The complete Pokemon RBY pokedex, part 9

Pokémon Name: Muk
Type: Poison
Classification: Sludge Pokemon
Pokédex Number: 089
Ability: Stench and Sticky Hold
Dream World ability: Poison Point
Useful Attacks: Gunk Shot
Location Found:
D/P/P: Evolve from Grimer
HG/SS: Routes 16, 17, 18, Celadon City
B/W: Poke Transfer

You'd think Gunk Shot's immense power would be enough, but no - it also stands a chance of poisoning the enemy as well. Not like they'll last long, particularly if they're Grass type. Poison's big drawback is that it's weak against five types yet only strong against one, so you might want to keep the Ground and Dark-type attacks that show up during Grimer and Muk's evolution.

Evolution: From Grimer at 38