The Devil Donut is a donut who made a minor appearance in the episode "The Donut of Shame."
The Devil Donut is a red donut, with red frosting, devil horns, and a red tail, who gives Patrick bad ideas.
The Devil Donut appear after Patrick steals SpongeBob's donut and interrogate Patrick of his actions. They appear to act like his conscience. The Devil Donut tells Patrick to not listen to the Angel Donut because "he's covered with sprinkles," while the Angel Donut tries to stop Patrick from hiding the doughnut 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 aforementioned 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.