The Mirror Monster was an evil magical duplicate of Shaggy Rogers, created when he stood too long in front of a magic mirror in Castle Revolta.
Physical appearance
The Mirror Monster is identical to Shaggy from the neck down. He shares the exact same outfit as Shaggy. His head, however, is a demonic version of Shaggy's; his eyes are spirals of red and yellow, his ears are tall and pointed, his goatee covers much more of his jaw, his hair is more frizzled, and his mouth is filled with pointed teeth.
Personality
The Mirror Monster has a sadistic personality in contrast to his mirror counterpart; as soon as he was freed from the mirror, he began following Scooby with intentions unknown beyond intent to harm. He seems to be somewhat naïve, as he was at first fooled by Scooby's mummy disguise, and like his mirror counterpart is also somewhat cowardly, shown when he fled immediately when Matches burned him.
History
Scooby-Doo and the Ghoul School
Shaggy, Scooby-Doo, Scrappy-Doo, and Matches entered Castle Revolta in search of the missing Grimwood Girls, and Shaggy and Scooby found the magic mirror in the entrance hall after splitting up from the others. Scooby-Doo saw his own evil reflection in the mirror and he ran away quickly to get Shaggy. Shaggy thought it was just a distorted reflection, like a fun-house mirror, but when he turned his back, the Mirror Monster grabbed him and took his place. Shaggy remained trapped in the mirror.
The Mirror Monster began following after Scooby, who ran away as soon as he saw that he wasn't Shaggy. Scooby briefly managed to fool the Mirror Monster by disguising himself as a mummy, but when the Mirror Monster saw through it, Scooby fled again, inadvertently jamming a mop bucket on the monster's head. Scrappy and Matches found him and, thinking him to be Shaggy, Scrappy removed the bucket, only to put it back upon seeing the monster's face. The monster removed it again, and it hit Matches as the monster grabbed Scrappy. Scrappy threatened the monster, who disregarded the threats up until an incensed Matches blew fire at his backside. The monster dropped Scrappy and fled, and for good measure, Matches burned him again. The monster immediately returned to his mirror, deeming the outside to be too dangerous, and dumped Shaggy back outside.