Her eyes narrowed and a frown upon her muzzle, Luna glared at Celestia unhappily. "Thou..." she broke off, unable to continue, than took a deep breath and started again. "Thou lets Daniel treat thou like a... like a pet? Like a... like a slave? Doth he become thy master, and wrap thou within chains? Beat and whip thou? We shalt kill him for such utter disrespect!" she thundered.
At her sister's harsh words, Princess Celestia shook her head, as she replied, "Oh, no-no-no-no-no-no... Lulu, you don't understand! When I say pet I mean it literally; I allow Daniel to treat me as a pony from his world; as a domesticated animal..." The Solar Matriarch lapsed into silence as her sister's expression turned thunderous!
"That..." Luna struggled to speak, "... that is even worse!" she screamed, almost lapsing into the Royal Canterlot Voice. "Tell us sister, doth he make thou eat hay? Doth he make thou sleep in our palace stables? Make thou... d-do thine business outside on the grass?"
At this point poor Celestia just couldn't take it anymore. "NO HE DOESN'T! LET ME EXPLAIN!" she howled, employing her own version of the Royal Canterlot Voice, causing Luna's eyes to widen.
"Then..." the Lunar Diarch uttered softly, "then explain it to us, sister. Tell us how he treats thou whence thou playest at being a..." she grimaced in distaste, "...a pet, for him."
Celestia nodded. "At the end of the roughest days of court," she began, "after we share the evening meal, Daniel and I return to our quarters. He knows when my days are rough; usually he has things prepared beforehand. Once we're in my chambers, I remove my Royal Regalia and then he and I enter the bath chambers..."
Stomping a hoof impatiently, Princess Luna growled, "And... what then?"
"Then... he bathes me..." Celestia replied, her mind awash in memory.
"Bathes thou?" the Lunar Diarch prompted. "We thought yon attendants helped thou wash..."
"Not anymore" Celestia replied. "He... Daniel sent them packing and now he bathes me himself! He washes my face, mane, and body, Lulu, cleaning the day's sweat and grime from me so... so very gently, giving me a massage at the same time. His hands and f-fingers can do things to you and reach places that pony hooves just can't and the feel of his touch upon my body... mmh..."
Her eyes wide - feeling a little flustered by the vivid images racing through her mind - Luna licked her lips before softly rasping, "And after he bathes thou... what then?"
"Daniel helps me from the bath and then dries me off with a large, wonderfully fluffy towel," Celestia answered promptly. "After that, we return to our bedchamber; there he... he caresses my neck with his hands as he puts my collar on. It's a simple thing, made of leather and fleece-lined - like a dog's collar - with a tag with my name on the front, and Daniel's Earth Address on the back. Once my collar's on, that's when I truly stop being Princess Celestia and become his pet, just... just Celly. We... we'll be on the bed together then, him sitting and me stretched out with my head in his lap. He... he feeds me, Lulu... by h-hand..."
Awed by the bliss suffusing her beloved sister's voice, Princess Luna swallowed thickly before she rasped, "Wh-What dost he feed thou?"
"Simple things for a simple pony" the Solar Diarch replied. "Fresh hay, fresh grasses, flowers, baby carrots and corn, and sometimes a special 'mash' of oats, grains and bran carefully mixed with this stuff from his home called molasses; it's like a very thick kind of syrup, sticky, tasty and warm..." Celestia licked her lips, overcome with the memory of Daniel feeding her those oh-so-sweet baby carrots and of eating a thick, sticky warm ball of the bran mash right from her beloved's gentle hand. "... And even as I eat, he's always petting me with a hand, stroking along my neck or combing his fingers through my mane..." she murmured.
Fanning herself with a wing - 'Is it hot in here all of a sudden?' - Princess Luna whispered, "And after that? What dost he do then, Celly?"
"He lets me roll around on the bed awhile; lets me kick my legs in the air and wriggle about 'till I'm comfortable... and then he really starts petting me, my jaw, my cheeks, my mane, my ears, my back, wings, and sides, m-my flanks, my hooves... and... everywhere..."
At this, Luna's eyes became impossibly wide. "E-Everywhere?" she squeaked.
Overcome with emotion, Celestia blushed like a school filly and nodded, an ecstatic shudder making her fall to her haunches. "Everywhere..." she whispered. "Oh, Faust, Lulu... his touch... mmh... his t-touch... I feel as if I've gone to join the Great Herd; as if I've entered the Elysium Fields... And then... then, when I'm so contented and sleepy I can barely comprehend a thing he'll have me move so we can get into bed proper. He spoons me, Lulu, did you know that? Daniel holds me so closely, an arm thrown over my barrel - or sometimes I'll sleep with my head atop his chest."
Squeezing her eyes tightly shut even as they misted with tears, Celestia whimpered as she fought to overcome the turmoil of emotion rolling through her. "His love... it overwhelms me; I could drown in it; I could die in it and it wouldn't matter at all because he makes me so happy, Lu. B-Before he came..." she rasped, "I... I was always so alone, so cold, Lulu. Now... n-now everything's so... so warm. I'm always warm!" Luna looked on as joyous sobs wracked her sister's body, Celestia smiling so honestly, so genuinely even as she wept, happy tears falling from her eyes, down her muzzle to drip unto the office floor. Her own eyes misty, a lump stuck in her throat, the Lunar Diarch was at that moment overcome by a little insanity - 'We... We want that!' - and made to race from the room in search of Daniel and such bliss!
Her efforts were thwarted, however as Celestia playfully tackled her down onto the plush carpet, "Noooo!" Celestia said, giggling through her tears. "You go get your own human, David, and show him what to do; Daniel's mine!"
"But... Daniel... more... experience!" Luna grunted, struggling to free herself. Celestia - Celly - just shook her head and tickled her beloved sister until Luna howled with laughter, giving her the occasional little peck on the nose! A short while later, the two sisters finished their brief tussle upon the office floor and then - after crashing on specially designed cushions - inquisitively, the Lunar Diarch asked, "Aside from all that, sister... what else dost thou do when thou art a pet for Daniel?"
Princess Celestia blink-blinked and then - a blush suffusing her features - she softly whispered, "S-Sometimes... Sometimes he... rides me..."
At this soft, hesitant admission, Luna goggled her jaw dropping, "Ruh..." she choked, swallowing thickly, "Ruh... Ruh... Ruh..."
Celestia giggled, covering her mouth with a hoof, as she replied, "Yes, dear sister, he rides me..."
"Ruh... Ruh... Ruh..."
"... Yes, sometimes in the way you're thinking... mmh..."
"Ruh... Ruh... Ruh..."
"... But mostly, he rides upon my back..."
At this, Luna snapped out of her shock. "Thou lets him ride thou... like a... likest a...?" she stuttered.
"Yes" Celestia replied, giggling again, "like a domestic pony from his world. We have a special set of traditional riding tack that we had custom-made just for me. He puts it on me oh-so-slowly, and then we'll canter about the palace, or the fields outside. It's..." here she trailed off, before starting again, her eyes alight with memory. "It's a release Lulu; the giving of control totally over to another being, having to trust that being to not abuse you; the knowledge that I can just let go - be a simple pony - and know that Daniel will care for me utterly and never hurt or abuse me in any way... it," she panted softly, licking her lips, relishing the thought, "it's... intoxicating, Lulu!" Lost in memory - unaware of her sister for the moment - she continued. "Sometimes... it would be so easy Lulu, to give it all up and become a pet pony forever. No more worries... no more responsibilities... Just the saddle upon my back, the wind in my mane as Daniel rides me... and then the warm bath at the end of the day followed by a rubdown and then... th-then... mmh..."
As Luna watched, half in awe, half in wonder, Celestia - overcome by some ancient pony instinct - stepped towards Luna 'till she was nose-to-nose with her beloved sister and then made a sound, deep and primal; she blew through her nose, "Are you a friend?" In the spur of the moment - not really knowing what she was doing, yet beckoned to do so anyway - Luna returned the blow, "I am your friend/sibling." At this, Celestia nickered quietly and nuzzled her beloved sister's coat; Luna did the same and then - guided by the old, old instincts, the two sisters nuzzle-nuzzled each other and began to groom...