The devil donut is a red donut with red frosting, devil horns, and a red tail.
Role in episode
The devil donut appears after Patrick steals SpongeBob's donut and interrogates Patrick for his actions. It and the angel donut seem to act like his conscience. The devil donut tells Patrick to not listen to the angel donut because it is covered with sprinkles, while the angel donut tries to stop Patrick from hiding the donut from SpongeBob.
The devil donut devises a plan in which Patrick hooks the doughnut to a chain, eats it, and can pull it back up with the chain before SpongeBob finds out what happened.
This is reminiscent of popular media nicknamed "Psychomachia," where a character's conscience (i.e: Good, telling good advice and Evil, telling bad advice) tells the character what to do in a certain situation.