The alien was a would-be conqueror of Earth.
The Richie Rich/Scooby-Doo Show
He created pod clones of Shaggy Rogers, Scooby-Doo, and Scrappy-Doo in order to get them on television to terrorize the earthlings into submission. Unfortunately for him, he made a poor choice of television programs, a show where "impossible" things are shown. He was defeated when a grand piano was dropped on his head (this was intended as a stunt by a daredevil performer and the alien got in the way).