My attempt to craft a scenario that would have a believable reason for a transformation from human to pony, after a discussion about how all TF stories were either tranny tier TG shit, or made no sense (The "you arrive to Equestria, and you immediately poof into a pony for no reason whatsoever" scenario.)
Might come in handy some day.

>Be Twilight.
>Be messing around in your laboratory with magic not meant for mortals to mess with.
>You're a mortal, so you probably shouldn't be doing this.
>But what's the worst that could happen?
>Suddenly, your experiment becomes unstable, and tears a hole in the universe.
>Oops.
>Okay, purple, don't panic.
>You got this.
>Just as you barely manage to start charging your horn, some kind of two-legged alien falls out of the hole with a scream.
>Crap.
>Okay. maybe you don't have this.
>Making a split-second decision, you start stabilizing the dimensional tear, as it looks like it's about to overload and rip the world in half.
>Deal with the alien later.
>It takes you a minute, but you manage to do it.
>Yay, crisis averted!
>No need to tell Celestia about this.
>"AAAAAAAAAAAAAHHHHHHHHHHH!!!"
>Oh, right. The alien.
>Whoops. Slipped your mind.
>You examine its form.
>Since it arrived here, it appears to have...
>Burst into flames.
>It's flailing around, sparks of supercharged magic falling everywhere as its body is slowly disintegrating.
>Oh dear.
>It even sounds like a colt's voice.
>You attempt to scan one of its appendages, but the moment your magic makes contact with it, it violently atomizes in a burst of Primal magic, which washes over you.
>Freaking out, you wish for everything to stop so you can get a breather and analyze the situation.
>To your surprise, it works.
>Everything stops, including time itself.
>You blink as you try to understand what's happening.
>It takes you a little, until it clicks.
>Primal magic! The very essence of Creation!
>The existence of it was only theorized, and archmages attempted to purify even a drop of it for their entire lives.
>And you're SOAKED in it.
>You can bend time itself to your will!
>But how?
>The alien...

***
>Be Anon.
>Get sucked into a different universe.
>Your alien matter doesn't agree with this universe's materials, and it causes some kind of matter-antimatter reaction.
>Not having a good time.
>Kind of hurts a little.

***
>Be Twiggles again.
>Weigh your options.
>With this power, you could do.. Anything!
>But at what cost?
>The life of an innocent?
>No, you have to fix this.. Somehow.
>You can't send it back to its universe.
>It would die immediately, with how half-disintegrated it already is.
>And you don't even know how, or where!
>And if you touched it with magic, it'd just die completely anyway.
>Biting your lip, you make a choice.
>There's only one thing to do:
>Use the primal creation magic and create a vessel for the alien's soul, one that would be able to exist in Equestria.
>Unfortunately, you can't recreate its body from the brief glance.
>The body you're most familiar with is that of a unicorn: yours.
>But it sounded male...
>Your brother?
>It'd have to do.
>Focusing, you begin to weave...
>Bone by bone, organ by organ...
>As you reach out towards the alien, more and more of its matter gets disintegrated, fueling your spell
>You really hope it's not conscious for this...
>And with each burst of power, you replace that energy with Equestrian matter.
>Capturing its mind and soul without disintegrating them takes some temportal trickery and manipulation to acctually achieve.
>But you're fairly certain you managed to do it.
>Once you finally finish, an uncoscious unicorn lies before you.
>A tall, lean, but strong unicorn. Elegant and alluring even while its splayed out on the floor.
>Practically a paragon of unicorn beauty.
>Did you overdo it?
>No, it's the least you could do after abducting and essentially killing it.
>No, not it. Him.
>You confirmed that was well when you looked back through time.
>As you study his features, you begin to get flushed.
>What's happening?
>...
>Oh. Maybe you made him a little too pretty after all.
>...
>Damn, those are some fine leg-
>Focus, Sparkle,
>You're going to have a lot of explaining to do...
>Both to him, and to others.