Rainbow Dash

There was no question about it. Something was definitely wrong with Twilight.
After we'd returned from Canterlot, she'd holed up in her new library, swearing not to come out until she was struck by inspiration.

"If I can't change things back, then I might as well help us along." she'd said. "I might not be a pony, but I'm still me. I still have my lab, and I still have a curious mind. I'm going to try my hand as an inventor."

However, lately she'd been hanging around the border of the Everfree Forest. In fact, she'd been hanging out all over Ponyville. She'd been hanging out everywhere except her lab, where she'd said she was going to stay until she invented something. When I caught up with her one day and asked her how the inventing was going, she looked at me like I was crazy. When I tried to talk to the others about it, they all just shrugged it off. All but Pinkie Pie. Apparently she'd noticed something strange about Twilight too. We decided we'd corner her together the day after tomorrow. However, Twilight had been acting really strange today, and I'd decided to find her and confront her, Pinkie Pie or no Pinkie Pie.

Something's up, and I'm going to find out what.

I found her in her lab - for once - with a strange glob of green goop.

"Heya, Twilight. What'cha doing?"

"Oh, nothing..." she trailed off, and for a moment it looked like she was trying to remember something. "Nothing, Rainbow Dash. Just testing this weird goop I found near the outskirts of town."

Wait a second. Did she forget my name? Nah, that's stupid. It must have been something else.

She turned to me, a strange, sinister look on her face.

"Wanna help?"

"I guess. What do I do?"

She turned back to the table, and the blob lying on top of it.

"Just stick your arm in."

"Um, are you sure that's a good idea? It could be all acidic or something."

She looked at me with a dark expression.

"You wanted to help, didn't you? Now stick your hand in the blob."

I approached the thing. It quivered slightly when my hand touched it. I pulled my hand away.

"I-I dunno, Twilight. Maybe this isn't such a good idea."

"Oh, for heaven's sake, just get over here!"

She lunged at my arm. I pulled it away.

"You're a tricky one, Rainbow Dash." she said, slowly advancing towards me. "Out of all the people I've fooled, you're one of the few that's actually seen through this little charade. Now that you know most of the truth, I guess it won't hurt to reveal my true self. "

Green flames erupted around her, and she started laughing like crazy. When they died down, a changeling stood in Twilight's lab. Even though the thing was trying to kill me, my brain, being the awesome thing that it is, took the time to notice everything about the changeling.
It looked like a pony, sure, but something was just off about the way its limbs looked, kinda like someone had tried to make a human but had gotten a lot of the bones in the wrong places. Its arms and legs were super skinny, and her fingers and toes were clawed like Spike's. She wasn't wearing anything, but she didn't have any... other features. Her face was blank as well; not even a mouth or nose, just two glowing, green, unblinking eyes. They narrowed.

"Now that I've shown you who I really am, be a dear and stick your hand in that blob of goo!"

I flew up to the roof of the lab, out of the way of her grabbing arms.

"Oh, so that's how you're gonna play, huh?" She said, walking calmly towards the table. "Very well, then. If you won't touch this thing willingly, I'm going to have to make you touch it."

At this, she picked up the blob of goo and hefted it, getting ready to throw it at me. I tried to dodge out of the way, but I'd been so shocked by her transformation that I'd failed to notice the spell holding me in place.

"What's the matter, Rainbow Dash? Can't move? Well, that's too bad. Don't worry; when I'm done with you, you won't be able to move for a long while, if not forever."

She threw the blob, the gooey mass hitting me on the wrist. It started to grow, going up along my arm, then up to my shoulders, until it reached my head, where it made a bubble. In no time, a thick cocoon had formed around me, trapping me completely.

"Congratulations, Rainbow Dash." the changeling said with a smirk. "You managed to put up an adequate fight. But now that I've got you right where I want you, one of the other changelings will be able to take your place. You're going to help bring our queen back."

Spike

From the stairway leading up to the rest of Twilight's house, I let out a small gasp.

Twilight's been replaced by a changeling! I've gotta warn the others, and fast!

I wasn't sure what the changeling had meant by 'bring our queen back,' but I knew that me and the others needed to act before it was too late.

I can't trust Twilight or Rainbow Dash.

I raced out of Twilight's house towards Sugar Cube Corner.

Chapter End

Author's Notes:

These chapters are meant to be pretty short, so as not to interrupt the main story too much.

EDIT: Given the usual treatment. I also added a description of the changelings in this world. I actually kinda like the way they look now.