The monster was an act put on by Frank, a man who lived secretly at Castle Von Dinkley, and who actually resembled a Frankenstein's monster.
He has white eyes, greenish skin, and an oddly-shaped head like a Frankenstein.
He acts menacing and chases after people with monstrous howls.
Scooby-Doo! (DC Comics)
Frank tried to scare potential heirs away from the castle, because Victor Von Dinkley convinced him that otherwise he would lose his home. He was captured early on, but his lack of a mask scared the gang and allowed him to escape. He was caught later after Velma found clues to his real identity. (Family Monster)