Que Horrifico is the fifth villain of Scooby Doo: Mystery Incorporated.
Daphne is babysitting a young boy named Arthur. After she puts him to bed, she hears strange pan flute music. Going up stairs to check on Arthur, she finds him turned into an evil monster. He chases her out of the house, and she sees the creature disappear.
In the morning, the gang sees the whole street is full of spookified kids who have driven out all the adults. Their parents tell them not to try to solve the mystery so they can turn the neighborhood into a tourist attraction.
The next night, another street is spookified and rid of adults. The gang visits a new South American professor and he tells them about the creature, and how its music can transform children into monsters. After another street is attacked, the gang sets up a trap at an abandoned house, and pretend to be a family that has just moved in. As predicted, Que Horrifico attacks, and they trap it, but in plays its song from under the net, and its evil minions free it and lay siege to the house. As the gang is cornered, Fred steps on the TV remote, and the children's favorite cartoon comes on. The kids gather around the television. The flutist tells them to attack the gang, but the kids take off their costumes and say that they want to watch TV instead. The gang chases Que Horrifico, who sees an approaching cop car, then trips on the traps ropes and flings a net onto itself.
The creature is revealed to be Mary-Ann Gleardan, a tweenage girl who is Fred's civics tutor. She says that she wanted revenge for being denied a position on the city council even though she is qualified in every way but her age. She learned about Que Horrifico in the South American teacher's class, stole the things for her costume from the school theater, and bribed kids from kindergartens and elementary schools with candy so they would go with her plan to rid the city of adults and become its new ruler.
Mary would have to if it weren't for those Hommetto Boys Cabuela (it actually meant Meddelesome kids in Latin).
The next villain is Alice May.