The Werewolves are the villains from "To All the Ghouls I've Loved Before," the pilot episode of The 13 Ghosts of Scooby Doo.
The backstory provided in the episode had the titular 13 ghosts placing a curse on the townspeople of Ursolvania. Upset at being captured inside the Chest of Demons, the ghosts placed the werewolf curse on the entire town.
The Mysteries Inc. gang (Scooby, Shaggy, Scrappy, and Daphne) crash landed in the Himalayas during a flight to Hawaii. Their encounter with the cursed townspeople first came when they were on trial after meeting Flim Flam, with the judge ordering them to leave by sundown. The group heads to a tavern where they meet Vincent Van Ghoul, who warned them about the werewolf curse put on the entire town. The mayor and his female assistant decided to keep the group from leaving, and they are served freshly squeezed wolfsbane, which only Daphne drinks, even after Scrappy's warning that the drink turns people into werewolves.
All the while, a singer going by the name, Ghoulio, performs, but with the full moon in the sky, he turns into a werewolf, much to Scooby and Shaggy's shock. All of the rest of the townspeople, including the mayor and his assistant, turned into werewolves as well. Daphne turned heel and became a werewolf herself, and after her villainous transformation, she aligned with the other werewolves against the rest of the gang. Shaggy and Scooby escape the werewolves and headed down a pothole, where Film-Flam and Scrappy were. However, the evil Werewolf Daphne followed the group, and continued snarling at them. At that moment, Film Flam splashed Daphne with his patented "Lotsa Luck Joy Juice," which turned Daphne back to normal. The rest of the werewolves went after the gang, and they were also cured by Flim Flam's potion, ending the curse.