Dr. Donut is a character and Candy Person from Adventure Time. He first appeared in the episode "Mystery Train," where he was tending to a murdered Candy Person. Finn accused him of murdering the Candy People, but then the lights flashed and he was nothing but a skeleton. At the end of the episode, it was revealed that he was one of the Candy People that played a part in Finn's Birthday surprise by turning into a skeleton.
He is a donut with his face on the side. He wears no clothes and has a medical kit with him.