The mummy was one of the many disguises of Zalia Z. Fairchild, an actor who had been secretly living at Mammoth Studios for years and was sabotaging the production of a movie because he didn't want the studio to be torn down.
It looked like a typical mummy, covered in rolls of white tape.
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It came after Shaggy Rogers and Scooby-Doo when they wandered into a pyramid on a desert set. It trapped them in a room and sent a mechanical lion after them, but it crashed into the wall and broke apart.