I know that I was determined not to write an anthro story, because I personally don't like the concept at all. But this? - that doesn't yet count as one for me, so not a problem.

A human Twilight from Earth wound up in Equestria and on the planet Equus through crazy circumstances aided by her pony self's experiments. As smart as the two of them are, naturally they immediately realized the possibilities that opened up for them and together they embarked on research. A way back for the human was quickly found and a stable connection between the two worlds established. But that was not all, of course. Just think of the possibilities of meeting an interdimensional self of yourself belonging to another species. How could one ever let this possibility pass? Especially if one was called both times Twilight and was the smartest of the world?

Weeks of research followed and now only one question remains for the human Twilight. How does it feel to be a full-fledged alicorn with its strong body and powerful abilities? If only she had dared to ask earlier...

Trigger: Body Possession (This still probably counts as Body Possession even though it's mutual, temporary, and safe. Just so you know); Estrus, correct anatomy (both briefly mentioned) and Merge/Fusion or partial body swap (depends on the way how you see it).

Artist the Story is based on is "nauth" on DeviantArt or Derpibooru. Image is called "Twilight Centaur" / ID:#1147764. It´s considered as mature and questionable for bare female human upper body. So there you go.