It was an exhausting afternoon for Derpy the Mail Mare.  She’d taken three breaks on her shift, but the heat made flying even short distances miserable.  Sure, in mid-flight there was the momentary chill of the wind whisking the beads of sweat off her soft grey fur, but the sensation never lasted very long before she had to stop at the next house.  Her mailbags were now empty of the few letters the Equestria Postal Service now trusted her with, so now began her daily task of retrieving improperly delivered packages.

She swooped down from the clear blue sky and, with a stumble, landed face-down in front of her first stop: Twilight Sparkle’s library.  She shook off the dust, adjusted her dark blue cap, and trotted up to the front door, clopping on the door with her hoof.  “Twilight Sparkle, are you there?” she asked.  

There was no answer at first, but then a rustling sound coming from inside and the unusual staccato of two clopping hooves instead of four.  Derpy tilted her head, her wild left eye squinting, when the door finally opened.  A musty odor wafted outward, making her nose wrinkle.  She took a step back.  

Even with her vision being what it was, Derpy could tell there was something different about the town librarian.  As it was, Twilight had transformed once more, stimulated to her bipedal form by her right hand which was sandwiched between her thighs at a spot not far from Derpy’s scrunched face.  She was leaning against the door, the fingers of her left hand wrapped about the frame, their grip tightening and relaxing.  All the while, Derpy kept sniffing, turning her head this way and that, trying to place the scent that now overpowered her.

“Can I help you, Derpy?” asked Twilight impatiently.  Derpy jumped, then saluted with her right hoof.

“Good afternoon, Twilight Sparkle,” she said, licking her dry lips.  “Sorry to bother you today, but I–I think I gave you something I shouldn’t have.”  Twilight giggled at the thought, her rubbing hand stopping its work.

“Well, I’m not so sure about that,” said Twilight.  She slipped the hand from between her thighs and lifted it to her face, the digits glistening with fresh juice.  Derpy began to sweat harder under the duress of the heat.  She stood entranced as Twilight’s mouth opened and she thrust both fingers inside, humming with delight as she sucked the nectar off of them.  She grinned as the digits withdrew with a quiet pop, her lips puckered in a kiss.  

Derpy was panting.  She was so thirsty!  The sight of the mysterious clear liquid, the abundance of which still trickling from between Twilight’s legs, looked so inviting, yet so unreal.  But she shook her head and dismissed it as a mirage.  Derpy guided the hat off her head, allowing her beautiful blonde mane to shake free.

“Can I have a drink of water?” asked Derpy, “it’s really hot out here.”  Twilight giggled again as she turned to her side, gesturing her inside with an extended right arm.

“Come in,” said Twilight.  Derpy perked up and trotted in triumphantly, setting her cap aside on a nearby table.  It quickly fell onto the floor.  Twilight breathed in the stink of the mail mare’s sweat as she walked by, not caring how loudly she did it, before her horn lit up and slammed the door shut behind them.  

It was the middle of the afternoon, but the entire library was dark, save for a few windows half-covered by wrinkled curtains.  Derpy’s eyes took an extra moment to adjust before she followed Twilight’s silhouette into the library.  Only a few steps in, Derpy’s hooves were sticking to the hardwood floor, not knowing that she was walking through puddles of Twilight’s smelly juice.  

In the dim lighting of the massive tree, she could still see sheets and blankets lying haphazardly on the floor, pillows piled in corners, and books and artifacts that had been thrown from structurally-compromised tables and into indecipherable heaps.  The edges of the wooden cabinets were noticeably worn, ground into a lighter shade of tan by some unknown force that had rubbed against them.  The disarray made Derpy feel uncomfortable.

“So,” said Twilight, standing in front of her with her hand back between her legs, “I’m sure you’re—wondering why I look like this, right, Derpy?”  Derpy looked confused, then her eyes widened when she remembered that, yes, Twilight suddenly had hands.  Twilight continued, her voice quiet and halting from the encroaching pleasure.  

“Well, it’s been a—fff—a real mystery ever since you delivered—this book to me,” she continued, lifting the copy of “The Goddess Within” with her free hand before letting it fall on a wooden chair.  “You see, last night, I was—touching myself like this—mmm—and—and I changed.”  She knocked the book off the chair and sat in it, rubbing her clit more feverishly as her legs splayed outward, her chin to her chest.  

“Then I had an orgasm, this—thing the book said happens when—you rub your marehood too long.”  Twilight gasped, grabbing her left breast as her head rocked back, shaking her mane as she did.  “And–and—UNH!” Twilight’s thighs tightened a large burst of juice squirted from between her fingers, the remainder dribbling from her pussy and down the front of the chair.  As her hands began to bulge into hooves once more, she jammed the growing mass into her splayed-open snatch, and the fingers returned.  “—And I turn back—to normal when it happens.”

Watching the juice dribble off the front of the chair and onto the floor made Derpy lick her lips.  Up close, it looked even more refreshing than before, a mix of tiny purple and pink stars floating in the secretions.

“Can I have that glass of water, Twilight Sparkle?” asked the mail mare.  Twilight only shook her head and stood up, raising one finger as she continued her lecture.  Derpy sighed.

“And, the next day,” said Twilight, pacing around Derpy, “I had sex for the first time—mmm—with Spike.  I had him put his hard cock inside me, just like the book taught me.  And he changed with me.”  Twilight hissed when her slender fingers tugged her hard right nipple.  Looking down, she walked behind Derpy, who was sitting patiently on her hind quarters, her grey ass partially glued to the floor by the secretions.  

“I’ve been experimenting with it ever since, finding new ways to let it happen to me.”  Twilight got on her knees and slid her hands up and down Derpy’s quivering front legs, easing the saddlebags off her back.  “And I am very, very good at it.”  She then opened her mouth wide and brought her lips to Derpy’s ear.  “But there’s one thing I haven’t tried yet,” she whispered.  As the words puffed against her face, Derpy could feel Twilight’s tongue slide up the edge of her ear.  “I want to do it with a mare.”

Twilight’s horn glowed a fierce purple in the darkened library, and Derpy felt herself being pushed forcefully onto her back.  She fell so quickly that she tried to get away, but she was now stuck fast to the floor, her wings rendered useless by the adhesion.  Derpy looked up and saw Twilight lean into her for a long, deep kiss.  It felt good to the mail mare, but she couldn’t figure out why, and her terror rose.  Twilight lifted her head, then leaned down once more to give her a peck on her chest, her tongue slipping out to tickle her soft, sweaty underbelly.  Derpy cringed and let out a high-pitched moan, a single tear falling from her eye.

“Please, Twilight Sparkle,” she said, “can I have something to drink?”  Twilight sat up.  She was annoyed at first, but her downcast frown soon melted into an eerie grin.

“Okay,” said Twilight, turning her body around and letting her cum-matted tail flick Derpy’s side.  She hoisted her left leg over Derpy’s chest, giving her a clear view of her trembling pussy as it squirted two drips of juice onto her chest.  As Twilight’s knee reached the other side, the soaking slit grew larger and unfurled, the purple ass descending rapidly as it quickly slapped onto Derpy’s face.  “Now drink, you dirty little pony!”

Her body roasting under the heat of Twilight’s loins, and her mouth wedged firmly between the folds of Twilight’s thick lips, Derpy opened her mouth to gasp for air, only to end up with it getting filled with a load of sticky cream.  Her body convulsed to the strange taste, and she gulped it down quickly, only to find herself coaxing more of it out with her tongue.  

Twilight leaned her ass back and Derpy’s nose was pinched shut, causing her to clop her hooves helplessly against the sticky floor.  With great difficulty, she managed to wriggle her nose free, a trail of the strange material shooting out of each nostril.  She heard Twilight moan loudly, her purple fur tickling Derpy’s nose as she rocked back and forth on her face.

It was then that Derpy felt her legs hoisted upward, each gripped firmly by Twilight’s wandering purple hands.  No sooner did the mail mare feel the strain in her spine before Twilight’s tongue dipped into her marehood, wriggling inside the tight confines and awakening sleeping nerves.  Derpy’s mouth widened, accepting more cream as her whole body began to tingle.

Then came a feeling that tightened, then slowly released every muscle in her body.  As Derpy’s body relaxed, a sense of mind-blowing euphoria took hold.  It felt like she was melting, her entire form spilling outward into yet another puddle on the floor.  Her four hooves stretched further from her body, fingers splayed from flattening hooves.  Her hips poured in all directions, trying to scissor her molten thighs together.  And as she melted, she could feel her soaking ass kiss the wood floor, the sensation of Twilight following her marehood down to the ground.  And when Twilight leaned forward to continue devouring her, her purple ass lifted, her breasts now squeezing into her lap.  

Only this time, Derpy had breasts, too.  And hands.

Suddenly, the events that preceded this moment no longer existed for Derpy Hooves.  It felt good having Twilight lay on top of her - relaxing even - and it felt like some kind of dream where everything was in slow motion.  Derpy gently brought her hands up to Twilight’s ass, kneading the imposing purple spheres until her tail whisked by her face.  She sat up a little and let her snout slip between Twilight’s buttocks, her tongue flicking the rim of her partner’s asshole.  She pulled back her face and gently slapped Twilight’s ass, letting it shake briefly.

“My turn,” cooed Derpy.  Twilight lifted her head, then rolled onto her back, guiding Derpy on top of her.  As she stood straddling Twilight’s face, she eased a strand of blonde hair from her face and admired her new body.  Two large breasts shook free from her chest, capping a long, sweeping curve that now made her as tall as Twilight.  And as she eased her throbbing pussy onto Twilight’s face, she rushed her fingers through the batch of twisted grey pubic hair, somehow carefully crafted into the shape of a certain baked good.  “Do you like muffins?” she teased as she eased down on Twilight.

Derpy moaned louder than before, her body falling, then snapping back to vertical, letting her blonde mane and tail float freely behind her.  Derpy rocked forward and back, thrusting vigorously over Twilight’s face.  Her wings quivered, then fanned out triumphantly, locking in that position as pounding blood flowed through their lengths.  She then swooped forward once more, placing her open palms on the floor, and walked her way down into Twilight’s lap.  A fresh rivulet of juice welcomed her, and she lapped it up quickly, flicking Twilight’s clit with her tongue before shaking her head back and forth to tease it further.  Twilight freed her face and moaned, releasing more of the tiny stars.

“Wow, you’re—really good at this,” she cried, her breaths growing more raspy by the moment.  Derpy beamed with pride, licking her lips before burying her face between the purple thighs once more.

Faster and hungrier they licked each other, juice pooling under Twilight’s body as their tongues grew numb from the effort.  Derpy’s back arched and she came first in one large burst, splattering cum all over Twilight’s face.  As she lifted her head to release her first orgasmic moan, Twilight writhed under her and squirted, gushing all over Derpy’s chin so that it dripped down the front of her neck.  As they continued to squirt on each other’s faces, the two changed ponies felt as though their bodies were merging, their legs tangled together even as they slowly slid back into their old forms.

Eventually, Derpy got her legs under her and she trotted off with her head held high, allowing Twilight to get back on all fours.  As Twilight recovered, the librarian couldn’t help but feel a little disappointed, wondering why the grey creature was being so cavalier about their encounter. Nevertheless, Twilight’s purple horn glowed once more and “The Goddess Within” floated into Derpy’s view.

“Here,” said Twilight, “you can have your book back now.”  Derpy caught it with her mouth.

“Hwank yew,” said Derpy as she nudged her way under her saddlebags and trotted out the door.  She tripped at the entrance, but calmly shook it off and flew away.

No sooner had Derpy left before Twilight lay back against the discarded pillows in the corner.  Splaying her legs apart, she stirred her tender pussy with her hoof, breathing in deeply to keep fresh the memory of the grey creature who rode her face.  Anxious to see if she could still transform without the book, she soon felt three fingers slip inside.  As her body blossomed forth once more, one question began to distract her from the pleasure.

“Why didn’t anypony tell me we could do this?”