For Rainbow Dash, the one advantage to a day this hot was that she was finally alone with her thoughts.  Her sleek blue body leaned against a tree, her forelegs crossed behind her sparkling rainbow mane, she exhaled in the shade as her wings flapped once, then pressed back against her sides.  

With a smile, she closed her eyes and dreamed again of the day she would finally join The Wonderbolts, particularly what it would be like in their dressing room as she stepped her hooves into one of those tight spandex outfits for the first time.  It was a fantasy she had fleshed out with every possible detail through countless repetitions, imagining it all the way down to Spitfire reaching under her chest to ease the zipper up her body, pulling the outfit taut around her every curve.  From that point on, she thought, she would be as inseparable from The Wonderbolts as the suit from her flank.

As Rainbow Dash dreamed, she also took in the quiet trickling sound of the nearby shallow brook, listening to the water bubble among the smooth rocks as it slipped downhill toward a distant bay.  Though it was flying she loved - and with it the freedom of racing higher and faster than anypony had gone before - it was water that fascinated her.  On a number of her many flights, she’d head out alone to the sea and fly as low as she could, feeling the salty spray against her face as she cleaved a trail across the rolling waves.  On a day like this without a cloud in the sky, a full afternoon of that was just what the doctor ordered.  And as she sat, she now felt refreshed, powerful, and more alive than ever before.

The more she listened, the more aware the pegasus became of her bladder, which she’d filled to capacity from an earlier drink.  She looked around her and took a moment to listen for hoofsteps, then relaxed once more, sliding down further.  As her tail bunched beneath her, she let her hind legs splay apart to reveal her small marehood.  Then, with a gentle sigh, she relaxed her loins, letting a narrow stream of urine squirt out of her in a wide golden arc.  

Urinating in public was frowned upon in Ponyville - especially after The Mare appropriated funds for indoor plumbing - but Rainbow Dash was not a mare to be denied her pleasures.  For twenty seconds or more, she continued to pee, letting the twinkling leak dribble onto her rainbow tail until the stream disappeared.  She liked the way her wee smelled -  she’d painstakingly experimented with different oats and grasses for that purpose - and she didn’t mind the warmth of the resulting puddle against her buttocks.  Sure, it made her tail smell strange, but it was her tail, and she knew she could do with it whatever she pleased.  After all, this was her true pleasure, her biggest secret, and she always took precautions to make sure it stayed that way.

And this was why she jumped when Derpy landed not far from her feet, her ass thwacking hard against the grass.

“Hello, Rainbow Dash!” yelled the mail mare, the front of her large hat flopping down over her eyes before she adjusted it back to its proper place.  Rainbow scrambled, pushing her thighs together in a hasty attempt to hide her shame.

“Oh, uh, hiya, Derpy,” squeaked Rainbow with a little wave.  “What are you doing here?”  Derpy trotted around to Rainbow’s left, then leaned back against the tree as well.  

“Oh, you know,” said the mail mare, exhaling.  To Rainbow’s surprise, Derpy closed her eyes and let her grey legs swing apart.  Urine sprayed out from her marehood, creating an impressive trail that extended more than three feet.  “Ahhhhh—”

Rainbow didn’t like the stagnant scent of the mail mare’s urine as it overpowered that of her own, but she couldn’t stop watching, either.  The more she did, the more the blue pegasus found her hooves begin to rub over her body.  She breathed deeply, feeling strangely anxious the longer she watched.  Eventually, her hooves nudged against her closed thighs, and after a few taps, she let them both wriggle between to reach her marehood.  But by this time she was doing so unconsciously, not realizing what she was doing until she tapped her dormant clit, sending an unexpected shock backward through her loins and up to the base of her damp tail.  Rainbow hissed, then looked back at Derpy, who was humming to herself as she continued to pee.

“You like to do that with your marehood?” asked Rainbow.

“Mmm-hmm,” smiled Derpy, her eyes still closed.  “I always come here to do this.  It feels so nice, you gotta try it.”  Rainbow’s hoof nudged her clit a little harder, and she felt her marehood start to get wet to the touch.  She found herself entranced by the spot where Derpy’s piss battered a patch of grass, so much so that she thought the sight alone was making her own marehood wet.  Quietly, she leaned closer and closer to the spot until she let go of herself and was down on all fours, her face just inches from Derpy’s puddle.  Then, in one quick movement, Rainbow flipped herself over like a dog, her marehood facing Derpy’s as she slid underneath the stream.

“Ahh-aaaaaahhhhhh—” moaned Rainbow as the mail mare’s hot water splattered against her chest, vaulting hundreds of tiny drips off of her and all over her face.  

In an instant, the proud pegasus seemed to lose control of herself completely.  She writhed in the puddle of stench below her outstretched wings and opened her mouth, letting the drops tickle her tongue as she tasted her.  Her front hooves vigorously rubbed her smelly belly, slipping and sliding over the urine as it beaded on her blue fur.  Eventually, her left hoof made it to her face, and she licked some of it off, tasting its bitter flavor as her other hoof squished over her now-soaking marehood.  She gave a low moan and her body tightened, her head grinding itself against the grass.

Unbeknownst to Rainbow, Derpy was watching her now, and she didn’t mind what she saw.  Nearly a minute had passed since she started to pee, and though her bladder was almost empty, she smiled as she watched Rainbow’s body begin to change, remembering her experience with Twilight.  

Rainbow’s legs and torso stretched at the same time, moving the terminus of Derpy’s stream to her belly button, then finally over the hoof that was rubbing her marehood.  As it did, her hoof seemed to melt under the heat of her piss, reducing the hoof’s width as fingers slipped free, then plunged quickly between the urine-and-juice drenched lips growing thick to her touch.  Above her hand, a narrow ring of rainbow-colored pubes formed an arc that terminated somewhere beneath her palm.  Rainbow moaned again, and two dark blue nipples popped up from her soggy blue chest, but very little fattened beneath them as only hints of breasts manifested there.

Only then, at last, did Derpy stop peeing, the mail mare shivering as the last few drops tickled her equally-stimulated marehood and landed on her tail.  She lay there and caught her breath, idly grazing her engorged grey lips with her right hoof.

At Derpy’s feet, Rainbow was utterly consumed by pleasure, continuing to rub her lubricated body and squeeze her tiny breasts.  As she shoved her fingers deeper inside her, she moaned even louder and twisted her legs, juice gushing down her fingers and into the grass.  The more Derpy watched, the more she began to touch herself, feeling the same sensations she felt earlier that day.

Derpy got on her hind legs and, supported by the tree, felt her own body begin to change once more.  Her legs grew first, causing her to lose her balance and just barely catch herself with her front legs as they pressed down on either side of Rainbow’s hips.  Her face now inches above Rainbow’s stomach, Derpy then slipped her hoof back to her marehood and continued to rub, hopping her stretching upper body forward on her other hoof as it morphed into a sticky hand.  The more she stretched, the larger her breasts became, compromising her balance once more.  Derpy bent her knees to crouch lower, and it was then that Rainbow began to feel her presence.  

But Rainbow’s vision only cleared just in time to see Derpy’s face above hers, blonde hair tickling the edges of her face as her lips pressed into hers in a kiss.

The two mares let go of their marehoods and Derpy let her weight fall on Rainbow, sharing the soaking stench of the urine on her body, their combined warmth roasting them both.  Wrapped in the embrace of each other’s arms, they rolled to their left, then tumbled into the brook with a loud splash, Rainbow landing on top of Derpy.  The two lovers found a soft spot in the rocks, a cradle of algae cupping Derpy’s ass in its slimy smoothness as the cool water rushed by her hips.  

Seeing Rainbow still straddling her, Derpy then felt the blue pegasus kiss her neck, her collarbone, then suck her giant left breast, grabbing onto it with her new hands as if trying to pull it off of her.  As Derpy again shivered to another’s touch, she lifted her head from the water, her soaked blonde mane whisking from the water and slapping against her back.  

Feeling drips of water against her head, Rainbow’s kisses accelerated, a half-dozen tiny pecks down Derpy’s torso all the way down to the waterline and her muffin-shaped bush.  She had just enough time to swing her legs wide when Rainbow got to her knees and reached under the water to hoist up her ass, allowing her to bury her face in Derpy’s snatch as water dribbled off her thighs.

Derpy could feel Rainbow’s voice humming, trembling her marehood to her swiftly lapping tongue.  Rainbow’s tail whipped against her flank as her asscheeks parted, making her reach back from Derpy’s thighs to ram three fingers hard into her begging slit.  Derpy’s hands fumbled to hold onto the slick rocks and she walked her upper body upwards, water dripping from her fingers as they rushed through the back of Rainbow’s glorious mane.  But she couldn’t hold this position for long before Rainbow let her ass splash back down into her rocky perch.  

Still in a rush, Rainbow sat down in the water and thrust her leg under Derpy’s thigh.  Then, with hands flat on the rocks behind her, she pushed back against the mail mare’s lap, bashing her crotch into Derpy’s so that a geyser of cold stream water sprayed up between them.  The two leaned back and moaned loudly as their marehoods kissed beneath the waves, spurts of juice squirting up onto their laps with every wet slap that followed.  Derpy tried to set a steady pace, but Rainbow pushed back harder and faster, water splashing all over their hot bodies.  

Rainbow’s blue body sweat profusely as she huffed deep breaths to ram her more than twice a second, her tiny breasts vibrating with every bump as the water disturbed into white foam around her.  Derpy was helpless to the barrage, and she stopped moving her own hips to let it happen to her, feeling the water caress her hips as it rushed by, her head swung back so her tongue could flip free, her heavy breasts flopping up and down.

Just when Derpy’s loins had almost become numb from the cold water and the constant banging, Rainbow cried as she slammed her once more and stopped.  Derpy could feel cream squirt out of her partner and into her own bruised pussy.  Then she pulled back and locked against her again, another geyser of water splashing them both as she pumped more of the juice inside her.  

Rainbow was about to pull back again when Derpy’s body reared back and bashed back at her the other way, unleashing a sizeable volume of cum that drenched the entirety of Rainbow’s loins, which then slithered off her body and was carried downstream.  The two then grabbed each other’s shoulders, trying to keep themselves from falling over, and their marehoods kissed once more, throbbing less and less as they released their final loads.

In a few moments, they each felt a hoof at their back, their bodies back in their normal pony forms.  They breathed deeply, inhaling the odor of wet cum and dried urine that covered them both, feeling the trickling water of the brook rush over and past their tired bodies, carrying some of their sex with it.

“So—awesome—” said Rainbow.  “I’ve never felt that way before—I like it!”  In a burst of excitement, she got on all fours and kissed Derpy on the cheek.  Derpy blushed.

“You’re welcome, Rainbow Dash,” said Derpy with a huge smile. “Twilight Sparkle showed me how to do it.”

“Twilight?!” said Rainbow, helping Derpy out of the water.  “That egghead likes to do this, too?”  Then she giggled.  “No way!  This I’ve gotta see!”