A Pup Named Scooby-Doo

S01E02 The Sludge Monster from the Earth's Core

At the Coolsville bank, a robbery takes place, and the janitor claims he spotted a monster in the vault. However, no one takes him seriously and he ends up being accused of the crime. His story gains credibility when Scooby-Doo discovers the monster hiding in his own doghouse, confirming the janitor's account.