My guess: It's NONE OF THE ABOVE!

I'm calling it early, they all know the tongue thing, and the first pony she visits scissors instead of doing oral. Onto the next chapter!

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Scootaloo.

Judging by it being featured in FlutterDash group first, then TwiDash group, my guess is it'll be Fluttershy.

Guessing on the first chapter for posterity- it's Rainbow Dash, having time traveled to bang herself so she'll go on to bang all of her friends.

How Mare Do Well could kiss, lick and suck while her mouth is under tigh putple fabric mask?

Which of her friends did she just fuck?

Dunno if I'd use that wording Rainbow, you were pretty much just laying there and taking it like a good filly

I agree with some of the other comments, I don't think it's any of her friends (which makes Rainbow going and trying to screw all of them all the funnier). It could be Discord for all we know (porn fanon does tend to give him a talented tongue), but I really don't think it's one of the other Mane 6.

For the fic itself, I think the foray into present tense is an interesting choice. I have to admit, since most people do it on accident while using the more common past tense, for the first bit of the fic my brain kept going into "editor" mode and thinking it was a mistake. But you were perfectly consistent as far as I could tell. I still think past tense is absolutely better in pretty much any writing situation, but you executed well anyway.

7429364 Because smut.

I want to say it's twilight, but maybe Rarity. Can't tell for sure.

Ah a more sexual version of what is actually my favorite story here called "The Games We Play by Absoluteanonymous".

I will be honest I hope it is the same character playing Mare Do Well.

In the science of deduction we can remove Twilight Sparkle as the suspect in question considering the story isnt over and there is already a chapter for her. Fluttershy could have some sort of assertion but I doubt skill is what she would have. Aj could be a candidate, there are many ships so she could be a suspect. Rarity could be lesbian, but in this context I doubt she is considering the fact that she is in to men in this current timeline. Our second possible suspect could be pinkie pie... Need I explain more?

Ahh she never did the deed no, that was all tongue swirling foreplay.

It's a random changeling. How else could she kiss and do other things to Rainbow through her mask?

That's right, Dash. That is definitely, totally the only plan that makes sense.

I think it is Flutters. I mean cum on (hehe) it's a smut story with flutters in it...... it usually flutters that's the kink in the sex link. Not that I'm complaining........ more plz.

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At this point, I've forgotten who it is myself! I should finish this sometime.

My guess is either "none of the above" (somepony borrowed the costume that was in the back of Rarity's wardrobe? Her sister, maybe, or one of her friends? Or anyone else who's heard the story. Luna?)
Or "all of them", taking turns.

Or she's going to rule out all of the possibilities, then convince Twilight to let her use a time travel spell to resolve the mystery, and finally realise it was her future self, because she knows exactly what works for her, and she's had a lot of practice over the course of the fic.