Uncle Beauregard was an uncle of Shaggy Rogers (or possibly a great-uncle or great-great-uncle, since he was in the Civil War). He bequeathed his mansion to Shaggy. When Shaggy went there to claim his inheritance, a villain dressed up as Beauregard's ghost to scare him away.
Scooby-Doo meets the Boo Brothers
Shaggy and his dogs discovered a trail of clues and jewelry left by Uncle Beauregard (a treasure hunt game with real treasure) leading to the main stash of gold and jewels in the fireplace.