~I am going to perform an experiment~ I hoof-signed. ~I am going to treat one of your Little Ponies as your ponies would treat a typical human. You need not worry; the pony who will be participating will not be harmed in any way. I have consent from said pony to do this.~
"I... I see..." Celestia stated quietly, nervously. "Who will be participating in this experiment? Not... Not Twilight, I hope?"
~No...~ I signed. A moment later, the doors of the throne room opened to admit the pony whom had agreed to my experiment... and upon seeing said pony, Celestia gasped softly before affixing me with a near-murderous glare. "No. No. Absolutely not!" she stated emphatically. "I forbid it! I..."
"Faust damn you sister, for once in your life, shut the hell up and let me do this!" Luna whinnied as she entered the throne room and then moved to stand beside me. "I agreed to do this, damn it; I want to experience this and you are not to interfere!" Upon her horn the Night Princess wore a powerful inhibitor to prevent her from using her magic, and around her neck a specially enchanted collar ensured that that only language she could speak was Ponytongue.
As Celestia looked on, her eyes misting over with tears, I asked Luna, ~Are you ready to begin?~
Luna nodded...
... and then, for the next ten years, the Night Princess was no longer. She acted as a wild pony and I began with gaining her trust. The part of her mind that was princess howled and screamed - this was undignified - but as Luna let go, that part of her mind was gradually silenced by a newer gentle, playful personality that began to emerge as she and I bonded deeply. You must understand: I was never cruel to her. I never shouted at her, or hurt her in any way. I was always there for her every day; I talked to her, fed her, groomed her mane, tail and coat, reassured her whenever she was startled or spooked by something, and soothed away bad dreams and nightmares.
Of course, there were bad days with the good; sometimes Luna just needed to vent; to decompress, to work out of her mind the thousand years of loneliness and pain she suffered when she was exiled to the moon by her sister. During those days, I just couldn't approach her as she galloped around and around the pastures set up for us by Twilight; as she snorted, kicked, bucked, reared and raged; as she whinnied and howled until her voice was - literally - hoarse. All I could do was employ my psionic ability "Charm" to attempt to soothe her rage. I also took great pains to ensure she didn't hurt herself. At the end of these fits of rage I would always be there for her, removing the sweat from her body then holding her gently as she wept, wept, wept, into my lap.
When at last I deemed she was ready, I showed to Luna - and then fitted her with - a halter to which I attached a lunge line; then I began her training. I trained her to follow my commands as she moved in a circle around me, introducing her to the words: "walk", "halt", and "back." I taught her to properly walk beside me while leading her with a lead rope - to walk, turn, stop, and respect my space as her master. Later, I introduced her to the sight and sound of her riding tack - a bitless bridle, a saddle and under padding, and a breast collar to ensure said saddle wouldn't move and had her become familiar with its weight upon her body. ~How's it feel?~ I signed once it was on her. ~I haven't cinched it, yet; we'll have you get used to the feeling of the weight before we start cinching it all...~
"Bah!" Luna nickered, "It's fine and it barely weighs anything at all!" Then, "Come on, come on, come on!" she whinnied, "Cinch it and let's go, go, go; this is what we've been working towards, master!"
~Are you sure?~ I queried.
Luna nuzzled and bumped me with her nose! "Yes, yes, yes, yes, yes, yes, yes, let's go!" she neighed.
~Very well,~ I stated and then - after cinching her tack and checking to make sure everything was in place, it wasn't too tight, and she was comfortable - I carefully mounted her and seated myself in the saddle. A moment later, a cry escaped me as Luna galloped toward the edge of our pasture and then took off, soaring into the sky! As we blew past the palace and soared out over Canterlot, through mind speech I asked her ::How do you feel?::
"Good!" she whinnied, boldly. "Clean. Pure. Joyful!" As we flew onward, I laughed heartily and petted her neck whilst beneath us ponies below stared and pointed at the bizzare sight of a human upon their Night Princesses back.
* * *
Later, we stood before a very uncomfortable-looking Celestia, Luna standing proudly in her tack, almost smug. She was honored to be wearing that tack! "I... I trust the experiment was a success, then Lulu?" When Luna nodded, the Solar Princess continued, "Good. I'm sure it was quite enlightening. You'll be wanting to return to your duties now, I expect. Luna? Lulu...?"
Luna, however, wasn't listening. She was gazing at Celestia, thinking of her former life. The little ponies who didn't love her. Raising the moon and arranging the stars night after night but nopony caring. Night Court, long and boring; the nobles constantly plotting and scheming. A sister who was too busy to spend time with her. Long, lonely nights where she would often weep alone, wishing for the sister or even the friend who would never come... Well... her friend was here now. Lulu knew where she wanted to be...
Celestia looked on in shock - her jaw dropping, her eyes widening - as her sister balked, snorted, and then turned away from her! Horror and despair gripping her guts, the Solar Diarch watched as her sister moved to stand beside David, nuzzling him affectionately. "I... I don't understand..." she whispered, voice thick with emotion. Then, a gasp escaped Celestia, the white mare paling as I stepped forward and spoke to her in Equestrian.
"The experiment..." I rasped, "was a complete success. Luna now knows where she desires to be, and you... you know the truth of utter despair... Most people are so grateful to have family; to have a sibling they can depend on--but not you, Celestia, you never were. And now... you will never be. Not anymore.”
With that said, we turned to walk away...
As within Celestia, the light of realization dawned...
Luna hadn't batted an eyelid when David had spoken...
Luna had known he could speak, all along, and hadn't told her...
Which meant...
The experiment had been planned right up to this moment; that Luna might watch her sister crumble...
Luna had wanted to abdicate her throne for a new, simpler life...
Luna had wanted to become a pet, in order to escape...
And David had obliged her; knowing that to watch Luna become said 'pet' would fill the Solar Diarch with despair...
"Y-You... you monster!" Celestia wailed. "Give me back my sister! Lulu, get back here!" With an inarticulate scream, the Solar Diarch lunged toward me - perhaps to impale me upon her ivory horn, but...
"WHINNNNNNNEEEEEEEEIGH!" ***CRACK!***
... she never got the chance, because Luna bucked mightily, kicking her in the chest and sending her careening back to crash into her throne. Seconds later, I mounted the former Lunar Diarch and then together, she and I fled. The last thing Celestia saw of us was my vicious grin as I rasped,
"Game over..."