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Original Description:

Attention, DeviantArt and Derpibooru! Coming soon to FimFiction, on either August or October: a new Equestria Girls movie called Monster Mash-Up Musical!

It is a special Halloween crossover movie featuring the Equestria Girls and the Grimwood Girls (The characters from Scooby-Doo! and the Ghoul School). It will air in October 2024.

The My Little Pony: Friendship is Magic/Equestria Girls characters are animated by Top Draw Animation, the Grimwood Girls are animated by Digital eMation (Sibella, Phantasma, Tanis, Winnie, Elsa, and Matches), and Elliott Animation (Galactica, Lagoona, and Minimal), and the characters voiced by the guest stars are animated by Tonic DNA.

Plot:
As Princess Twilight Sparkle and her friends - Spike the Dragon, Pinkie Pie, Applejack, Rainbow Dash, Rarity, and Fluttershy (including Starlight Glimmer and the Great and Powerful Trixie) to Nightmare Night in Ponyville, Sunset Shimmer comes and invites Twilight, Starlight, and Spike to go to the human world for Halloween (the human version of Nightmare Night). As they go to their the Crystal Mirror, Sunset, Twilight, Starlight turn back into humans, Spike turns into a puppy, they reunite their human friends from Canterlot High (except for Twilight’s human counterpart, Sci-Twi, who is away on holiday). The Girls felt ready to do some tricks, but they didn’t think costumes were scary enough anymore. The Girls felt that ordinary Halloween costumes might not be convincing enough. So, they get a special visit from Timber Spruce, the human world Twilight Sparkle (Sci-Twi)’s boyfriend from Camp Everfree. Timber made a ray machine that can transform any human into any specific creature. He was going to turn the Girls into REAL monsters - for REAL. They head for the vacant Canterlot High gym, as he thought it might be the most appropriate place to do the process, and with a press of a button on the ray’s remote, the ray activates and Twilight and the Girls (except Spike, who watches with Timber) actually get turn into REAL monsters.

(Spike is the only one who didn’t get transformed by the invention; he was perfectly fine wearing his skeleton costume.)

Even though they’ve been transformed, the Girls still feel, think, and act like their normal selves; even after Halloween, they can still turn back into these selected monster forms at will and also turn back into their true human forms. They were more than excited with their new monster forms and couldn’t wait to test them out for Halloween. After seeing a monstrous threat involving a horde of demons and succubi, the Girls (still in their new monster bodies) go to the world of Monsterdom and meet the Grimwood Girls - Sibella Dracula (vampire), Phantasma (ghost), Tanis (mummy), Winnie (werewolf), Elsa Frankenteen (Frankenstein), Enid (from OK K.O.! - as a witch), Galactica (alien), Lagoona Black (daughter of the Creature of the Black Lagoon), Minimal (Godzilla’s daughter), and Matches (dragon / pet of the Grimwood Girls), who are celebrating figuring out Sibella’s part of a will that leaves her with a hidden treasure that belonged to a late old southern male human, Richard MacWright, that she saved from drowning in a lake a few years ago. The human-turned-into-monster Rainbooms teams up with the Grimwood Girls, and new characters like a motorcycle daredevil human named Henry, a young world-traveling adventuring sorceress named Zoe, and Zoe’s 3 companions who are Wild Cats: Jason the jaguar, Tahlia the tiger, and Leonard the lion, to defeat 2 new enemies: a corrupted human accountant named George Sullivan who deeply hates the Rainbooms because of having been dumped by his girlfriend one summer for being their biggest fan at the time, effectively eliminating his shot of becoming a cop, and an evil warlock named Scarface - the leader of the demons and succubi - who wants to take over all of Monsterdom. The Rainbooms and the Grimwood Girls start working together to defeat the 2 evildoers and travel all across Monsterdom to find MacWright’s treasure.