Yuck

Yuck is a zombified version of the Pokemon, Muk. It evolves from Moulder at Level 38. It is one of the two zombie evolution lines available in the wild, other being the Graveill line.

Pokedex Entry:

Type: Dark Sludge Pokemon

HT / WT: 3'11" / 66.1 lbs.

A creature composed only of the mingled blood of MUKs and humans, YUCK eats as many people as it can to replenish its body, which it leaves behind it in a trail.

Appearance
Yuck is essentialy just a bloody version of Muk, but it has spikes added to his back, and has a child's head in its mouth.

Trivia

 * Oddly, Yuck is made of blood, while Moulder is made of toxic sludge. it is also still called the "Dark Sludge Pokemon".