Harkness and Who in Equestria.
Prologue
"Doctor, what's going on?!" Jack asked nervously as he was flung to the floor of the TARDIS. They were crashing, that much was clear. The Doctor frantically pulled levers and pushed buttons trying to steady the TARDIS. Jack pulled himself off the floor and clung to the railing. "Why are we crashing?" He asked.
"I haven't the slightest!" The Doctor said, "The vortex appeared out of nowhere. I haven't got a clue where we're going to end up!"
"That doesn't change the fact that we're crashing!" Jack yelled. The Time Lord looked up from the control panel.
"What? Oh! Yes!" He pulled a large blue lever and the TARDIS steadied, making it's familiar landing sound. There was a bright flash of light and Jack found himself on the floor again.
"Oh my head..." He mumbled, rubbing his head. He tried to stand, but found himself rather unsteady. He fell back to the floor and noticed for the first time something horrifying. He had hooves. He stood on four legs, desperately trying to keep his composure. "Doctor?" He called out. "Doctor where are you?" He didn't see the Doctor. He did see a horse-like animal staring at him curiously. It had brown fur, and a dark brown, spiky mane and tail. It almost reminded Jack of... No, it couldn't be! Could it? It was staring at him, it's eyes wide.
"Jack?" The animal said, sounding suspiciously like...
"Doctor?" Jack asked uncertainly. The creature smiled hugely and began to laugh.
"Jack! We're ponies!" He laughed. "I mean, this is impossible, you don't just change species. But still, brilliant! Positively brilliant!"
"Doctor," Jack said, a small smile appearing on his face.
"What?"
"You have an hourglass on your butt." The Doctor looked at his flank, eying the mark curiously.
"Yes... Yes I do. Take a look at your own flank Jack." Jack looked at his flank and found a heart, with the symbols for male and female on it. 'Figures,' Jack thought to himself while rolling his eyes. But that wasn't the only thing Jack found.
"Whoa." He murmured as he stretched out the dark blue wings. "I've got wings!" He laughed. "That is just... So cool!"