Dr. Croaker is the villain in "The Schnook Who Took My Comic Book", an episode of A Pup Named Scooby-Doo. He was the disguise of Wendel McWendel, based on a villain from the Commander Cool comics, which he created.
Dr. Croaker resembles a large purple frog, dressed in a physician's uniform.
Dr. Croaker hated Commander Cool and went around eating the hero's comics. However, he only took first-edition comics, which was an important clue.