The strange pony.

"What are we supposed to do? Just sit here?" Twilight asked. Applejack had told Twilight and Spike what had happened. Twilight started walking to the door. The castle was big, though. "The mare HAS to be tired. If you actually looked at her, she looked like she had been using magic for days!"

"But why's the questin'. Who were those stallions? And why were they chasing her?" Applejack asked. The question went to the side when somepony knocked on the door to the castle.

"I'll get it!" Spike ran to the door. "Yes?"

"I need to know if the mare in this picture has been through here lately." A blue-maned gray coated unicorn stallion was at the door.

"TWILIGHT?!" Spike called, nervous.

"What's wro-" Twilight's sentence was cut short, for that stallion in the door was holding a picture of that exact tan mare that they were talking about seconds ago.

"Oh, Princess Twilight, Ms. Applejack, my apologies.  I didn't realize that you were there." He bowed.

"How do you know this pony?!" Twilight asked, incredulous.

"So you've seen her? You've seen Dusty?" He asked, excited and nervous.

"Yes, but how do you know her?" Twilight asked, restating her question, punctuating by poking the picture. He bowed his head. (In respect, shame or guilt, she couldn't say.)

"Sh - she's my sister, your majesty."

Meanwhile...

Rainbow followed Dusty. That's the nickname she'd given the mare. She flew past the guards, into the castle. Rainbow apologized for her and flew past also. What happened next astonished her. Dusty almost blazed through the guards posted outside of the throne room. Almost.

"Hey! You can't go in there!" The guard shoved her down.

"But I must see the Princesses!" She ever so innocently replied. She held the guard in her gaze. Rainbow gawked at Dusty.

"Sorry, mister. My friend here is confused. Thank you for being so patient with her." And at that Rainbow turned Dusty's face to hers. "What were you trying to do?!" Rainbow shouted at Dusty. Her eyes turned green again, and she looked around like she didn't know where she was. When she realized, her eyes grew in fear and she ripped herself from Rainbow's grasp; she looked like she was running for her life. "Hey, wait!" Rainbow called after her, then flew after her.

"No, NO! Let me be!" She ran to the castle doors. The scene was beautiful. Celestia's sun was slipping down, painting pinks, oranges, yellows and purples on the sky. Innocent ponies going inside the many buildings, oblivious to the conflicted pony gazing upon them. A tear streamed down her face. Sadly, one day soon her walls will break, and her 'ghosts' would give them horror beyond any that they have ever known. She collapsed, and broke down in tears. Rainbow ran to the sound. Rainbow saw the pony that she would've flown across all of Equestria for, that same pony, crying on the floor. Rainbow didn't know what to do. She tried something that her friends would try. Dusty felt a hoof on her back. She looked up at Rainbow. A shuddering sigh escaped her lips. "Don't." Rainbow quickly raised her hoof. A bit of blue dust swirled around Rainbow, but she didn't notice. "Why are you trying to be my friend?" Sniff.

"You are such a mystery to me. I find you fending off Timberwolves in the Everfree, you're a earth pony, but you can fly and use magic, you're running away from these formidable stallions, and you even surprised Celestia and Luna. I wanna know who you are, and why you're doing the things you're doing."

Dusty sighed. "I'm not a pony, so you wouldn't and won't understand." Rainbow was confused by this.

"If you're not a pony, then what are you-" Rainbow gasped in realization. She'd inhaled the bit of blue dust. Her eyes tinted slightly blue. She backed away from what she saw as a changling. She then promptly stopped, then tried to pounce on the aforementioned changling. She fell on air. Dust swirled around her. A hoof pinned her down, and Dusty whispered to Rainbow. "I'm not what you think I am, nor could you even ever guess, for I am the only one of my kind now, and I made sure of it." Sigh. "I want to be left alone, and that's hard enough without somepony following me across the whole of Equestria." The hoof left. Rainbow jumped up and turned as quick as she ever did. She saw what almost looked like a dusty Tantabus. "You need to leave me alone before my fate finds me. And yes, my name is Dusty." Dusty stated. She turned solid, then zoomed towards Ponyville. Rainbow chased her, wondering who or what she was, and if Dusty was as big a threat as she acted.