The mouse was an ordinary rodent at Shaggleford Manor.
Shaggy & Scooby-Doo Get A Clue!
It ate a nano-formula-enhanced Scooby Snack and gained super-speed. Robi smashed holes with a hammer all over the house trying to kill it. It was still alive, dragging away a Bag O' Chips, when Shaggy Rogers and Scooby Doo last saw it.