Derpy was quite certain she would never get used to time travel with the Doctor. One day her life was normal with deliveries and other ponies laughing at her and then here she was bouncing (it was never a smooth trip) through time. The way the Doctor piloted the Tardis reminded her of when she had first learned to fly, but still, you couldn't argue with the results. So far she had helped saved Equestria from a Cyber-mare uprising, met Princess Celestia as a little filly and then kept her from being erased from time by some odd pony called the Master and visited a planet four million years in the future populated by pink monkeys with only two legs. It had been an odd week to say the least.
There was just something about the Doctor, something that made it easier for her to focus and believe she might not be the screw up everypony else believed her to be. Derpy liked that feeling and never wanted it to end and had resolved to follow the Doctor to the ends of time if for no other reason than she felt useful and, dare she think it, smart.
Now she was watching this mad stallion run about the controls of the Tardis randomly turning dials and pushing buttons. Every now and then he would get a frustrated look and kick the console, but otherwise his joy was infectious and Derpy found it impossible to not giggle a little watching him dart about. Eventually they came to rest with the characteristic whine of the Tardis’ engines and the Doctor finally regarded Derpy with a broad smile. “Here we are! Vega 34-niner and the year is 546 apple 3!”
Derpy had no idea what any of those words meant in that particular order, but the Doctor seemed very proud of them so she gave him her best smile. “And what are we going to find her, Doctor?”
“No idea. Never been here before, but the brochure seemed nice.” He stomped his front hooves on the floor and the door creaked open filling the room with extremely bright light. “Allons-y!” He didn't wait for Derpy and rushed out the door.
Derpy shrugged and followed him in to the blinding light. She was somewhat surprised to bump right into the Doctor who was staring at the Tardis. Only it wasn’t the Tardis she had just stepped out of, but an exact copy parked in front of them. Derpy looked back and forth between the two blue police barns (whatever exactly those were) and except for a few very minor dings and scratches they were identical in every way.
The Doctor’s look of confusion was a memory Derpy would always treasure as he looked at this duplicate Tarids. “What? What?! What?”
“Well it is a time machine, right? Maybe you come here again later.” Derpy was shocked she had spoken up like that and winced involuntarily waiting for someone to tell her how stupid that was.
The Doctor looked back at her and smiled. “You are brilliant, good thinking, but no. It’s impossible to cross my own timeline like this. Hold on… where are we?” He looked around and they both realized they were in a vast, white nothingness. Derpy didn't have time to become afraid before the Doctor clopped his forehead and smiled. “Of course, we’re still in the time stream only the two Tardises! Tardisuses. Tardi? Funny I never thought to pluralize that. Anyway they got too close and overlapped. That can happen sometimes, probably won’t destroy all of time, but should only last a few minutes. Or days, you never really know.” He moved closer to the other Tardis and scanned it with his weird screwdriver, but really Derpy wasn't so sure it was a screwdriver as she had never seen him use it on a screw. He studied the results and scrunched his face in concentration. Derpy watched the Doctor work smiling at the only pony to ever treat her as something more than a menace and wished she could make herself tell him how much it meant to her. She wondered if it would always be like this. Mad adventures, scary monsters and weird timey-wimey stuff until they grew old? That was fine with Derpy if it meant she could stay with the Doctor.
Derpy was so lost in thoughts of life with the Doctor that she hadn't even noticed the door of the other Tardis opening. A stallion trotted out, but not the doctor, not this was a completely different stallion. And yet he had the same cutie mark it was just everything else that was different. Mane, coloring and eyes even the tie he insisted on wearing was different. He smiled brightly at Derpy and rushed toward her. “Derpy Hooves if I didn't already know this would be happening I would say ‘what a surprise!’” He clopped himself on the forehead. “Oh of course, I had not explained this to you yet! Oh I do apologize for that, but as I recall I’ll be making it clear right now.”
Derpy was confused, like another joke was about to be played on her that she wouldn't understand and she hated that feeling. The Doctor looked pained, “Oh Derpy, I am sorry. Sometimes things get moving and I forget what I have and haven’t said. I’m a Time Horse, from another planet, and from time to time when bad things happen I can… regenerate. I take another form instead of dying. And this is a version of me from sometime in the future.”
Instead of being shocked, this actually just seemed to be okay with Derpy and the first question to jump into her mind was: “So does that mean there’s an older version of me inside too? Do I get a grey mane?”
Now it was the future Doctor’s turn to look pained. “Uh well actually, no, you don't travel with me any longer.” It was at that moment a young, pretty purple unicorn exited the future Tardis.
Derpy felt her heart snap. The purple unicorn smiled awkwardly an offered a hoof. “I’m Twilight Sparkle. The Doctor doesn't talk about his other companions much, but you must be Derpy Hooves. He’s said some very kind things about you.” She giggled nervously when it was obvious that Derpy wasn't going to shake her hoof and then slowly lowered it back to the ground. “I uhm… I'm pretty new to all of this time travelling stuff, but the Doctor tells me you were a quick study. I don't suppose you have any pointers for me?”
Derpy just shook her head still staring somewhat blankly. She wouldn't be with the Doctor forever. Not only that, but he would replace her and she had apparently replaced somepony else. It was a lot to take in and she began to feel more foolish than she had ever felt before.+
