Dark Rye Bread is the baddest bread, throughout the series. Whoever eats it becomes evil. Consuming too much of this bread will slowly turn the eater into a snake with each bite. The first 2-3 loafs with give him sharp teeth and a forked tongue. It had a major role in the episode "Bad to the Duck Bone".
SwaySway ate this bread and became a "bad boy", in order to win Jenny's heart. After eating more of this bread, he became very evil, turning himself into a snake.
The Dark Rye Bread's name sounds like Darkrai from Pokemon.