Just to be clear, this is occurring after Starlight Glimmer's reform.
Starlight Glimmer snickered as she slipped into her outfit. A land of equality and harmony for all? She had thought, My homeland calls for me...
As for her costume, she had created a full-body spandex suit, laced with tendrils of "magic" that she had gotten her hands on from some teens selling "magic in a jar", shortly after a recent school camping trip. Said magic would theoretically allow her to bypass the transfiguration that would normally occur as she passed through the portal.
Why she wanted to keep her human form? Simple: Fingers. And toes. Toes are always nice.
Either way, she was going to remain human, whether the creatures on the other side liked it or not.
It was a weekend, and aside from the usual sports meets that were occurring nearby, there was very little activity. On top of her current disguise, Starlight donned the identity of a janitor that had frequented the area. The thick, rubbery face, the awkward tufts of hair, even down to the sly goatee were all present on her mask.
And with her disguise on, the girl slipped into the portal.
"OOF!" Starlight bellowed, quickly covering her mouth with her hands as she recovered from faceplanting in the portal room of Twilight's castle.
"Who's there?" A frighteningly familiar voice called back. The sounds of hooves echoed throughout the room as the pony counterpart of Starlight neared the doors.
"NOBODY!" The girl shouted back, immediately cursing herself afterwards for making herself even more obvious.
The hoofsteps froze, before quickening as the pony grew closer and closer to the doors. Starlight's eyes widened as the handle of the door lit aglow, and slowly began to turn.
Frantically, the girl dove into an upturned box, quivering underneath in fear of discovery.
Meanwhile, the pony Starlight proceeded to shrug, and leave the room.
Walking on all fours is always such a pain in the a--
"NO SWEARS!" Pinkie screamed, landing in front of the girl from who-knows-where, "BAD STARRY!"
The mare promptly left Starlight with a smack on the face with a newspaper. The mask she had been wearing earlier was nowhere to be found.
"Starlight?" Twilight asked as she landed with a flap of her wings, "Why are you walking so... uh, weirdly?"
It was at that moment in which Starlight realized that she had been walking not only on all fours, but walking on all fours with her bottom held proudly (mindlessly? suggestively?) in the air, as a human walking on all fours would normally walk if they didn't bend their knees.
"And why does your plot look like a badly sewn dress?"
"And why are you both in front of me and beside me?"
Crap. The human Starlight thought, She's onto me.
The pony Starlight facehoofed. "Twilight," she deadpanned, "that's clearly the human me, trying and horribly failing to pass herself off as me."
"Ah, yes..." Twilight nodded, "Even though I'm not Rarity, I'd recognize that potato-sack sewing anywhere."