The Scooby-Doo impostor was the disguise of M'Lady Moonbeam, a smaller dog wearing a Scooby-Doo suit with stilts.
Physical appearance
The costume's physical resemblance to Scooby-Doo was perfect, other than having no collar and a harsher expression.
Personality
The impostor could not talk, and was much fiercer than the original. This gave the game away to Shaggy Rogers.
History
What's New, Scooby-Doo?
Season two
M'Lady Moonbeam unmasked.
She was helping her owner, Meadow, who dressed up as the Cat Creature, to kidnap the Secret Six puppies from a dog show. (Homeward Hound)