This fandom must be smaller than I thought, since no one has done this crossover.
Inspired after the events of the MLP:FiM episode "Magic Duel"/"Magical Duel" (it goes by either title) where The Great And Powerful Trixie returns with her new badass magic and mainly wants Twilight out, but she goes to Zecora (she's an evil enchantress-and she does evil dances...) that inspired me to write a "wrong spell" fanfiction with The Caster Chronicles and My Little Pony G4, Friendship is Magic. All translations are from Google Translate.
Enjoy!
(I do not own My Little Pony or The Caster Chronicles)

"Oh, come on- it doesn't look like it's gonna hurt anything!" teased Ethan, looking at the words written in Latin. "Just because it looks harmless doesn't mean it is." replied Lena, studying the page carefully. Her eyes narrowing to try to read it better. She looked up from the pages to her boyfriend "I can't make any sense of it."
Ethan sat down on the floor of her bedroom, studying it.
This had been their pastime for a while, along with various magic-related subjects. In the past few years, Ethan Wate had met Lena and her family of Casters, Incubi, ect.-, which were mainly a different type of witch, wizard, vampire, or some other mythical being. At first he expected her Uncle Macon to stand in front of the doorway when he came over and shout "you shall not pass!", but eventually Macon got wind of the joke and now Ethan had the tendency to nervously look over his shoulder now and then.
"I can't read Latin, period." he said, giving up. Lena giggled and went back to looking at it carefully. After a few minutes she pulled Ethan over towards her, pointing at various words. "I have some of it- see that? it's magia- magic." she said, pointing to the first word of the first line.

Ethan rolled his eyes playfully. "Whoa. 'magic' in a Caster book. I'm shocked."

She ignored him going on with her translation. "Terra, Caelum, et overlords- the translates as 'Earth, Sky, and overlords'" Ethan chimed in "so the first line is ' Magic, Earth, Sky, and overlords", whatever that means." Lena shrugged. She turned to face him, as if about to ask him a question. "magica liberalitas, benignitas, fides, officium, risus" she stated matter-of-factly, waiting for Ethans response. He scratched the back of his neck, completely clueless. "umm...magic...generosity, kindness, honesty...loyalty...and ….laughter?" he asked, and Lena nodded.

She read another line. "ex meis corpus humanum disputem..." her voice trailed off as she wrote it down in English on a sheet of notebook paper. "from my human body I plead-" he looked confused "this is getting a little weird...", she said as she picked up the book, repeating her last few motions. "pedibus me et libera me- Ethan, I think I've got it!" she said, throwing the scroll down with excitement, picking up the piece of paper as if it was a winning lottery ticket.

"and it says-" he asked, looking at her from the floor. He was a Wayward, not a Caster- so he refused to even read the cast, just incase he'd be transported into another dimension, or spontaneously combust. Whichever came first.

Lena stood up tall, reciting from the book and her notes.
"magia, Terra, Caelum, et overlords
magica liberalitas, benignitas, fides, officium, risus
ex meis corpus humanum disputem
pedibus me et libera me

magic, Earth, Sky, and overlords
magic, generosity, kindness, honesty, loyalty, and laughter
from my human body I plead
give me hooves and set me free-"

There was a crack, a sickening and frightening crack, like lightning striking inches from your face or hearing your skull crack on linoleum or concrete. Black smoke filled the room, circling them as they both were pushed by an unknown force to the ground.
As quickly as it had came, it vanished and as soon as both of them had been able to see -and think- straight, a voice echoed from downstairs.

"Lena Duchannes! You have some explaining to do, young lady!"