Local mail pony Derpy was standing with Dr. Hooves when the trouble started. The light grey pegasus knew she was considered something of ditz by the other ponies, but that didn't stop her from seeing that something was very wrong here! She backed away from the commotion, blonde mane swishing around her head as she shot the good doctor a questioning look.
"Good heavens!" He exclaimed. "There are so many of them among us already!"
"What are we gonna do, Doc?" It didn't even occur to her to question his identity. She knew him.
"Ah..." His brow furrowed in concentration, but his expression remained troubled. "A message!" He exclaimed at last. "You are a mail pony, so we shall send a message for help! I just need something to write with!" He nodded to himself and began rummaging around in his saddlebags.
"Oh." Derpy puzzled over this for herself, alarm rising in her chest as the tumult grew closer to them both. "Um, maybe this isn't the best place to write it, though..." She glanced around, wondering if she could pull the Doc into a building, anyplace safer than where they were now.
Th doctor's face lit up in what she recognized as his 'Eureka!' expression. "Yes, yes, here we are..." He drew forth a pen. "Hmmm... I was sure I had some paper as well..."
Two changelings leapt in front of them, causing Derpy to hop backward into the air with a yelp of surprise. Dr. Hooves looked up briefly, muttered something that may have been 'oh dear', and went back to searching his bags. The bug on the left addressed them, "Hey you two! If you're changelings, take your original form! Now!"
"Uh..." Derpy landed at the doctor's side. "We can't. We're sick!"
"Sick?" The changelings looked at each other and laughed. "Haha, get 'em!" They pounced, but were unexpectedly thrown back by a blast of energy!
Dr. Hooves blew smoke from the barrel of a wicked looking little contraption, a cavalier grin on his lips. "No time for a letter, I suppose?" He shook his head with a sigh. "Do fly away, my dear friend. I shall deal with these rapscallions!"
"Oh, Doc." She floated up into the air and gave him a little salute. He waved to her, changelings already closing in on him thanks to the noise his weapon had made. Derpy turned and picked up speed. She heard the weapon fire only once more, and then it was silent. Her heart sank, but she pumped the emotion she was feeling into her wings. Dr. Hooves and all of Ponyville could be saved if she returned with help in time!
Chancing a glance back over her shoulder, she saw multiple sets of glowing eyes following her in the darkness. "Gotcha!" A pair of hooves closed around her waist suddenly! Barely visible in the inky night, one of them had snuck right up next to her!
"Yeeeek!" She spun and flailed in a panic, fetching the changeling an accidental blow right in the face. He groaned and lost his grip, falling away from her. She saw two of the following pairs of eyes dart down to catch him.
"Careful!" One of those still pursuing her called, "She's using some kind of crazy aerial martial art!"
Derpy quickly straightened out and resumed her flight, knowing that it was very important. But... "Wait, who?" She called back to her pursuers without stopping.
The same changeling blinked at her. "No, I said martial art!"
"I don't know any Marshal Art!"
"Yeah right, we just saw!"
"What? Where?"
"Right here!"
"Well I don't see him!"
"Wh...!? Y... I..." The pair of eyes she was talking to seemed to be falling further behind, glow dimmed by confusion. Sadly, Derpy didn't notice the pair that had flanked her during the discussion. "Now!" Something sticky sprayed across her wings, pinning them to her body. She fell into the waiting hooves of the changelings, who then began towing her back to Ponyville.
Just before they covered her mouth Derpy cried out, "Save me, Marshal Art!"