"Well, duh," Rainbow replied. "You don't time travel every day. It's like, safe for me to go out there, right?"
Luna frowned and pouted slightly, tilting her head to the side. "She is a dream, yes," she said to Pinkie and Rainbow, the real one. "But I've rarely seen such a lively one. And never one manifest this way. Still..." She shook her head. "All dreams end. She's your dream, Pinkie, and she will disappear once you wake up."
Pinkie frowned a little at that. "Does that mean she'll be gone?"
"The same way all dreams are gone when morning comes," Luna replied. "They are just fantasies. Illusions. Products of your own mind and imagination, only outside of your control. She is no different from the thousands of dreams you've been through and forgotten in your life." She smiled towards Pinkie. "In a sense, she will never stop existing. She is a part of you, a product of your subconscious, and she will continue to be."
Pinkie was still frowning, uncertain.
"Can we get a move on?" the dream said. "I have things to tell Twilight, and we need to get there fast."
"And what is it you must tell her?" Luna asked, without looking at her. "Tell me in detail, so I may tell her directly myself should you be caught up in something else."
The dream didn't answer. It appeared stuck, caught between a desire to do so and some mental impossibility preventing it from. Then it flickered, visibly so. "I have to tell her myself," she replied, seemingly unaware of the pause she'd gone through. "We can't risk-" She froze, going completely motionless.
Luna held up a wing, keeping her like that. "See?" she told Pinkie. "She's not complete. She can't be. She is a dream, and though she is also a rather peculiar phenomenon I will like to study in detail at a later time, she is not any more alive than a fantasy you might have during the day is." She stepped forward, and lifted Pinkie's chin with her other wing. "And she won't be any more dead than those same fantasies are when you distract yourself from them."
Pinkie looked at Luna, then at the Rainbow Dash frozen in place. "I understand," she said, her voice calm. "Can I say goodbye to her, though?" she added.
"Of course." Luna smiled again. She lowered her wings, and the dream shook herself, only mildly aware something had happened.
The real Rainbow Dash looked around with some impatience. She felt like she'd heard some more rustling in the leaves around them, but she knew it was most likely just her nervousness playing tricks on her. Still, it didn't hurt to be careful. "Princess, we should go. Pinkie should not be here."
"You are right," Luna acknowledged her. "Do say your goodbyes, Pinkie."
The dream still nervously buzzed her wings. "Yeah, I'm with me on this one, we should be going. By the way, you said this is a dream, right?" She turned to the other Rainbow.
Dash nodded. "Yep. Why?"
"Did anyone check if my body came back too?" she asked. "I don't want to go back to the future when I wake up." She scratched the side of her head, thoughtful. "How did I even end up inside a dream? Hmm. I must have fallen asleep while going back." She nodded to herself. "Yeah, that ought to be it. I hope my body landed somewhere safe, then." She turned to Luna. "Come on! Wake me up, I have things to tell Twilight."
Pinkie approached her dream. They were both starting to fade, only subtly at first, but Rainbow and Luna both had enough experience to tell. "Goodbye, Rainbow."
The dream quirked her eyebrows. "Hey. We're seeing each other in just a few minutes." She looked down at herself, suddenly very confused.
Pinkie smiled at her. "It was fun knowing you. Maybe I'll see you again sometime." She stuck out a hoof, pausing a moment to observe its transparency, then gently waved. "The world is still all in one piece here, no floating islands and no swarms of building-eating fish."
The dream looked at herself disappear, and felt herself disappear too. But looking back to Pinkie, she also smiled. "I got it," she said. "I've already changed things. We've already changed things!" She smiled even brighter, and turned towards the pony she resembled. "My future doesn't exist anymore, it looks like. You take care of this present for me, alright?"
Rainbow Dash chuckled at that. "Will do," she said with a smile, and she gave her replica a salute.
The dream kept on smiling, almost fully invisible and colourless, and looked again towards Pinkie. "Goodbye," she said, no more than a whisper. Then she was gone, and Pinkie was too.
Luna looked at Rainbow. "It is time to go for us as well. You'll tell me about the fight you had in the waking world, I will reach you shortly. It's been a long night."
"It has," Rainbow agreed. She took a deep breath, and she too began to disappear, much more quickly than Pinkie and her dream had. Much too quickly to hear any more rustling of leaves and branches, though neither did Luna herself hear it. Then she opened her eyes, awake again.