Pinkamena was minding her own business inside Pinkie Pie's mind, only speaking whenever her other side wanted to chat or if something interesting happened, before she was ripped out of the life she was used to living and was dropped in an entirely new world, with a body of her own no less. Due to some injuries sustained in her fall she has a fateful encounter with someone who she later learns goes by the name 'Muzan', an event that changes her life once again and sends her on a quest to save this world in her own way, all while seeking to find the figure that had changed her life and learn more about him.

(Kimetsu no Yaiba (Demon Slayer)/MLP Crossover)
(Part of the Demon War story series, but reading the others isn't required)

Grogar, the Father of Monsters, manages to escape from his prison, but instead of returning to Equestria, or any part of Equus for that matter, he finds himself in a strange new world, aboard a large boat that is in the middle of a voyage to a new land, and with a new body. However, he finds that something is preventing him from leaving this world and returning to the one he came from, forcing him to band together with a lone Handler that has no A-Lister Hunter to team up with and a lone Palico that has no partner, in order to survive in this new world and learn about it. From there he will have to learn about the dangers of the New World, adapt to the monsters that call it home, and find out why the massive beast known as Zorah Magdaros headed to the large island in the first place, along with any secrets that might be waiting for him and his new allies.

(Monster Hunter World/MLP Crossover)
(Part of the Demon War story series, though reading the other stories is not required)

Starswirl and Autumn Blaze, both seeking adventure and understanding, find themselves transported to Veldin, a planet located in a remote corner of the Solana Galaxy, as infants, with no memories at all, and happen to arrive at the same time that Ratchet crash lands into the planet. Together the three siblings grow and experience all sorts of things during their time on their home world, but eventually it grows boring and they set their sight on the stars, imagining what sort of adventures would await them once they left Veldin behind and started to explore their galaxy... unaware that fate was going to bring all of them into contact with someone who would set their lives in motion and allow them to truly discover their places in their vast universe.

(Ratchet and Clank/MLP Crossover)
(Part of the Demon War series, but doesn't require reading the others)

Timeline
Ratchet and Clank: Complete
Going Commando: Complete
Up Your Arsenal: Complete
Deadlocked: Complete
Size Matters: Complete
Tools of Destruction: Complete
A Crack in Time: Complete
All 4 One: Complete
Into the Nexus: Complete
Rift Apart: Complete

Amaryllis, an infiltrator for Chrysalis' hive, searches Canterlot for her missing queen, but during her nightly visit to the city she is forced to flee and heads into the Everfree Forest, where she finds an unusual cave, one that ends up changing her fate and knocks her out in process. When she wakes up she finds herself in an unknown world, in a snowy land that rests at the northern end of the continent, in a body that isn't what she's used to having. From there she must relearn the skills of an infiltrator, discover how the people of this new world interact with each other, and either figure out how to get back to her home world, or get used to living in her new environment and killing people to make some gold.

(Skyrim/MLP Crossover)
(Part of the Demon War story series, but reading the others isn't required)
(Warning: involves Daedric Prince induced cow transformations and unusual theories to explain lore)

Twilight, Starlight, Cozy and their brother Jak, along with their friends Keira and Daxter, spend their days playing around the village, helping the other residents of Sandover whenever they can, working on their hobbies, and occasionally learning from Samos, the Green Sage that calls their village home, even though Daxter often gets him and the others in trouble whenever they do anything interesting. That doesn't stop them from having fun and learning a few skills to defend themselves, all while unaware of the grand adventure that they'll be embarking on in the near future, along with the fact that the fate of the world depends on their actions to come.

(Jak and Daxter/MLP crossover)
(Part of the Demon War story series, but reading the others isn't required)