Scooby-Doo and Scrappy-Doo (1979)

Season 2: Episode 5

The gang are visiting the Tibetan Mountains and come across a Cat Man and his gang of evil felines. The Cat Man throws Shaggy, Scooby and Scrappy into the shrinking water fountain reducing them to the size of mice. In their tiny form, the three manage to eventually push the Cat Man into the shrinking water fountain and then escape. Once away from the Cat Man's hideaway the three return to their normal size.