According to the robot Spooky, he was a fully grown Great Dane, with brown fur, having a strong family resemblance to Scooby. He had a menacing appearance, but that might be just for the robotHe wore a dog collar that had the initial, "D," as well as a black top hat and cape.
Scooby and Shaggy Rogers were both afraid of him; Scooby said he was the family weirdo.
Scooby-Doo (Archie Comics)
Scooby-Doo had been left the mansion in Uncle Spooky's will, and when he and the gang checked it out, they found it haunted by his supposed ghost. In reality, this was a robot built by Wolf Whippet who was trying to keep Scooby away, while he searched for a jewel-encrusted bone.