Magic mirror

The magic mirror appeared as a fictional character in at least one common panto.

As a result, an old "magic mirror" prop was one of the props stored in the understage area of one theatre where the Thirteenth Doctor, Yasmin Khan and Dan Lewis came to see Cinderella. After the Doctor used a link from Spacebook in an effort to bring "the magic of pantomime" to the performance, the props and costumes were replaced with real versions, "snatched" from other places and realms; as a result a real version of the mirror, leering malevolently, was summoned. It attacked the group alongside several other pantomime villains. After the Doctor found a Genie's magic lamp among the props, she was able use one of her three wishes to "wish" her friends safe, resulting in all the pantomime characters being returned to whence they came. (COMIC: It's Behind You!)

Behind the scenes
The Genie mentions in It's Behind You! that what the Doctor did "snatched" him from another realm. It is not clear if this was specific to the Genie, or if all the pantomime characters were summoned from parallel worlds as well, rather than being native to the Doctor's universe.

The is a character from the fairy tale Snow White. Its depiction in It's Behind You! is reminiscent of the design used in the Walt Disney animated adaptation of the film, though its toothy smile is unique to Russ Leach's depiction, with the Disney magic mirror instead being spookily impassive and emotionless.