Dr. Croaker was a giant, anthropomorphic male frog-like creature with purple skin. He wore a white doctor's coat with a pink shirt and red tie underneath. He had a stethoscope around his neck.
He hated Commander Cool and went around eating his comics. However, he only took first edition comics, which was an important clue.
Powers and abilities
He had a long, prehensile tongue, which he used to eat comic books, among other things.
A Pup Named Scooby-Doo
Shaggy Rogers had bought one of the few remaining first editions of Commander Cool from Selma when he found himself being menaced by Dr. Croaker, who attempted to steal his newly-bought comic book. He and Scooby-Doo fled and joined up with the rest of the gang.
Dr. Croaker showed up again and stole Selma's only remaining first edition of Commander Cool. He then confronted Shaggy and Scooby again and attempted to eat their comic. They tried to run, but Dr. Croaker caught Scooby with his tongue and ate him. However, Scooby used a jack to pry his mouth open and escape. Dr. Croaker continued to chase Shaggy and Scooby around until they used their "Commander Cool bouncy boots" to escape him.
Shaggy and Scooby got their comic book autographed by Wendel McWendel and then tried to leave, only to encounter Dr. Croaker again. He ate their comic, but Shaggy tickled him with a feather, causing him to spit it back out. Dr. Croaker chased Shaggy, Scooby, and the rest of the gang until they seemingly lost him in the basement, but Croaker was hiding in a dumpster. He stole Shaggy's comic and ran off with it.
Since McWendel's first edition was now the only one left, people were offering huge amounts of money for it. Once he agreed to sell it, he asked the kids to watch over it while he retrieved the keys to the case he kept it in. Dr. Croaker showed up and tried to steal the comic, but Scooby took it back from him. Croaker retreated, and Velma Dinkley spilled juice in his path so that he would step in it and leave orange footprints. The prints led to the dumpster in the basement, where Scooby found stilts. Velma came up with a plan to catch Dr. Croaker.
The gang informed McWendel that Shaggy had found another first edition of Commander Cool, meaning McWendel wouldn't have to sell his comic after all. Dr. Croaker swung by on a rope and stole the comic, but it was a trap. Scooby pressed a detonator, causing the phony comic to explode into a sticky, gummy mess, trapping Dr. Croaker. He was unmasked as Wendel McWendel, who tried to make a fortune by stealing everyone else's first editions so that his would be the only one left. He needed the stilts because he was so short. McWendel was hauled off to jail shortly afterward.