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Why not, let's do Mega Muk.
blah blah blah stats, raise Spec. Def and Spec. Atk by a moderate amount, make him Poison/Dark, Human-ish form much along the lines of good ol' Clay Face, but to keep it pokemonish, make him a little fatter and shorter.
New ability, Deadly Nightshade: Pokemon who become poisoned take twice as much poison damage per turn.