The Skull Ghost was a disguise used by T.J. Buzby.
He appeared to be a skeleton with a long cape.
He was willing to do anything to get the treasure, even push Shaggy off the roof of the mansion.
Powers and abilities
While impersonating his brother, Sheriff Rufus Buzby, he was able to reach the mansion without raising suspicious. He also used a ghostly robotic horse and rider to further scare Shaggy.
Scooby-Doo meets the Boo Brothers
He wanted to drive Shaggy Rogers away to give himself a clear field to find the Beauregard treasure.
T.J's plans were troubled with the appearence of an heir to the fortune. Thus, he turned to the disguise of the Skull Ghost. Thinking it better to let others do his work for him, he shadowed Shaggy and Scooby-Doo as the followed the clues, showing up just often enough to terrify them.
As the Skull Ghost, his plans were complicated by Scrappy-Doo, a runaway ape, and the scheming butler, Farquard. As things progressed, he became more desperate, going as far as pushing his foes off the mansions roof and tying them to a leaky boat. Both times they were saved by the Boo Brothers.
In the end, the Skull Ghost got the last clue. Rushing to the fireplace, he pulled the lever. The treasure came rushing out and trapped the would-be theif. Rufus, the true sheriff arrived in time to unmask his brother and arrest him.