he must have missed something, his mood darkened.

his stallionhood had taken note of the of the lewd noises.

Sweat beaded Fenris’ brow, a dark misma clouded his vision, and the air burned his throat

Miasma

Restraining his ire, he tossed the therimoire onto his bed and stomped back into his wash room.

grimoire

sure, it might have been a little weird to be eating out his best friend, but it wasn’t the strangest thing he’d ever done.

If that’s not the strangest thing he’s done, then what is?

As he stared at himself, drinking in all his features, he scrunched his snout in disdain. He’d lost his build, his height, and his roguish good looks in the blink of an eye - replaced by a short, soft, svelte parody of his former self.

Maybe he should have stopped using dark magic once he had everything he wanted, because if you mess up even once...

Steadily spinning around, momentarily compelled to issue a silent prayer to the dark lords below, he closed his eyes, took a deep breath, and slowly exhaled. It would have been mortifying enough to be stuck with the body of a lithe, little femcolt, although he knew that was far from the worst fate that could have awaited him.

Even a femcolt is still a colt, so it could certainly be worse for him.
Edit: Maybe I should have read the next paragraph before writing this...

“Dude,” Pixel shouted, issuing another hard knock, “are you ok in there? I heard something weird then a bunch of screaming! You didn’t kidnap that Rainbow Dash cosplayer again, did you?”

Fenris: "Don't worry about it. It's the real Rainbow Dash this time."
Pixel: "Understandable. Have a great day."

Mare or not, Fenris was his oldest and closest friend, and he’d never do anything to her unwillingly.

Just wait a while and the unwillingness will solve itself.

Good premise!
I look forwards to see where it goes.

So far the Kirin did very good. Bring his buddy pleasure, but not going overboard...

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Grimoire is typically used to refer to a book about magic without any further specifics, while therimoire is usually referring to a book of black magic in particular. It's pretty uncommon and I've only seen it used a few times.

damn this story has 100 comments

I don't care that I wasn't meant to, but it fells too much like rape. I'm not reading any further. I didn't even finish this chapter, just wanted to share my thoughts.

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It is, but I think that was the point.

I actually thought it would be worse and that would cause me to drop it but I plan to read more.