Princess Luna—Ruler of the Night and Mistress of the Moon—had been locked in a gilded cage and tucked away in a lonely corner of a dusty castle. Her only companion was a singing harp who had once been a pony named Lyra, or so it claims.
Every now and again the lord of the castle, a great oaf of a dragon, would come by to check on his collection and demand the harp play for him a song, to which it would always answer: “I would, but I haven’t any hands.”
Then he would demand that Luna lay for him a silver egg, and each time she would say to him: “Ponies do not lay eggs.”
“Selfish birdie!” he would then say. “Just wait and see. You will lay an egg for me.”
He gave her water and kept her fed, and though the portions were small the harp would give it’s share to Luna, proudly declaring: “Harps do not drink or eat.”
One day the dragon returned with a toothy grin on his face, as though for the first time ever he had himself an idea. “If harp can not play, and birdie can’t lay, then birdie shall play the harp for me.”
“And what if I object?” asked Luna cautiously.
“Then by my fire such pain shall be felt! A bird shall I cook and a harp I shall melt!”
There were no more objections, though Luna did ask: “What song would you have me play?”
The harp answered for him. “Just follow my lead. As I sing, you’ll pick up the tune.” The golden harp looked the princess up and down and sighed. “A word of warning and an apology, for I am a magic harp after all. This song you shall play, then an egg you shall lay, for this song is ‘The Birth of the Moon.’”
Luna was unsure of what the harp meant, but saw no other choice. The pony princess then readied her wings. When the harp began humming she plucked at the strings.
The song was sad, yet pleasing to hear. The harp sounded lovely, strings and vocals both. Luna knew this song from some forgotten portion of her childhood, and with the harp’s guidance she found it almost instinctual to play. Throughout the song and growing more intense all the time was an unnatural pressure within her body that seemed to strain against her bladder.
All at once she could play no more and fell forward onto her chin, the harp still singing unaccompanied, as though to punctuate this moment.
Luna’s mane and tail flared, drifting upward like dark flames. The princess’s body contracted and a silvery shape appeared within her winking pussy. Her eyes rolled back and she let out a moan that underscored the song as the egg stretched her wide, straining her clitoris until pleasure matched pain and her mind was cleared of useless thought. Then there was relief, of a kind so powerful and so grateful that it left her reeling, sucking down air in heaving gasps as the egg emerged, the greater width at last crossing the threshold and allowing the clenching muscles to clamp down on the sloping end, forcing it out and into the straw.
The music stopped. The egg was indeed silver, but of a milky sort that glowed in the candlelight.
“Now that egg is fine, just look at it shine!” said the dragon, enraptured by its odd radiance.
The princess’s pussy was surely not meant to lay eggs, and had never been stretched like that before. While she was still out of breath the dragon opened the cage and took the egg. It felt heavy in his palm, with a certain nightly chill that seemed to come from within.
“Birdie Princess really came through. What in return might this old dragon do?”
The harp begged the princess to ask the dragon to turn her back into a pony, but alas her curse was not so easily broken.
“Sorry, harp, it is not up to me. The curse’s caster is who you must see.”
Luna thought about how to play this. She hoped to ask for their freedom, but reasoned that it would not be given so easily.
“If not that, then how about you let us go.”
The dragon thought this over for a moment.
“My dear princess, I am loathe to let you go. Who else could give me such a wonderful show?”
“Hear this then, we shall play you one last show, but to sweeten the deal I’ll… I’ll let you fuck me while we do.”
The dragon was taken by surprise, then let out a laugh. “A curious offer, but you are in luck. I’ve been in need of a pussy to fuck.”
Luna began playing the song once again, and the nervous harp sang as she had back then. Soon Luna began to feel the magic egg slipping into her womb. Whether it came from her own ovary, growing as it slid into her uterus or was summoned through some wormhole she did not know.
She kept playing even as she was removed from her cage. Her eyes constricted as her squishy, magical pony princess body was stretched and pulled, fitted tightly over the tip of the dragon’s ribbed cock, then pulled down until her ass touched his testicles and the tip of his cock touched the shell of the egg.
After only a few physics-defying strokes the egg was coated in sperm, and he pulled her off his dick and placed her down gently on the windowsill.
Cum gushed out between winks, and with it popped the egg which began to glow as before, only this time cracks began to form in the shell. The egg, it seemed, had been fertilized, and soon hatched into a creature of fire and moonlight which soared into the skies, serving as a beacon to guide the prowling army of Celestia, seeking both her sister and vengeance on her captor.
Author's Note
I wrote the first draft in one sitting, then decided I was going to to change it. A month went by, and then another… 
Anyway, I got around to it eventually, whatever it is... 
