Rut.

The activation of nearly unstoppable biological impulses, cranking a stallion’s desires up to eleven, and drowning out any semblance of logic or reason.

He knew that his body was surging all sorts of earth-pony magic and male hormones and sexual chemicals to drive him to breed.

And he couldn’t stop it. He didn’t know if he wanted to stop it.

“O-oh… oh, big bro.” Cookie whispered, ears folded back in submission, her eyes dipping as she looked up at his feral, aggressive posture.

Caisson stamped again, snorting hard enough to scatter bits of hay that were strewn on the barn floor. His massive cock throbbed beneath him, slapping against his belly — his little sister looked up at it in a mixture of fear… and total desire.

Caisson watched her eyes flick from his dripping, throbbing stallionhood up to his face. He could see the excited-anxiety in those eyes: a swirling mix of that fiery sexual need, of a scared filly overwhelmed by a stallion in rut… and of an intense, little-sisterly love.

“Yes.” She breathed, clattering to her hooves and getting up on all fours, and turning away from him. “Yes, BC.I want this. I want it so bad, big bro.”

*CLACK* *CLUNK* *CLACK*

Caisson’s heavy clattering hooffalls were as sharp as drumbeats against the old oak floorboards, which creaked and groaned under his muscular size. His hooves were huge, so much bigger than Cookie’s as he walked over to her.

Cookie was panting now, looking back over her shoulder at him as he approached. With a little nicker and a turn of her head, she scampered forward, an excited-scared skip that brought her to the stacks of hay bales on one side of the barn, built up into a multi-level pyramid structure for later consumption. His little filly lifted her front hooves up onto the edge of the first bale, as if she were about to start climbing the structure to get away from him.

But then she tossed her head back again, that wild mane flowing through the air as she glanced over her shoulder again.

He could see that fire burning in her eyes, smoldering embers of lust fanned into an inferno that could consume the Everfree.

“I’ve been aching all day.” She flicked her white tail up, so high that her back quivered with the effort — she wanted him, her stud, to see everything. Despite the singular, primal drive thrumming through his rut-activated mind, the sight still made Caisson pause, flare his nostrils, and whinny.

Caisson let his eyes slip down the long, white strands of tail fur, to where they mingled with his sister’s taupe dock, currently flagging hard for him, wagging ever so slightly with excitement, like a happy puppy. The delicate underfur of her dock quickly ran down to a dark, almost pitch-black flesh ring, a muscular little ponut of flesh that quivered tightly.

Her tailhole.

It was so small and slender, just like her. Barely wide enough that it’s outer edges touched the athletically muscular pertness of her butt-cheeks. His gaze continued down, over that ring and across the desolate little patch of skin nestled between her tailhole and fillybits, which dripped with the humidity and heat that was rising from just below.

That’s where her plump fillyhood started. Those pitch-black lips looked so impossibly compact and tiny while paradoxically being puffed and engorged with his little sister’s arousal, all at the same time. He could see that she was tight, a small filly to begin with — and despite her pussy’s best efforts to expand and engorge, she was still a young teenager.

Her lips had parted slightly, pushed open by her permanently exposed and erect love-nubbin. Cookie’s clitoris was even bigger than it had looked when it was grinding along his shaft. The bit-sized heart-bud of swollen, nerve-stuffed, nectar-dripping flesh protruded out from the bottom of her teen fillyhood, a bright-pink signal that had peeked open her labia.

Through the parted folds, Caisson could see glimpses of his little sister’s tight, twisting pink interior. Her swirling inner-lips quivering and spasming, sometimes even betraying hints of the volcanic velvet tunnel within.

And then, as he was taking this in, she winked.

All at once that tiny, tight, teenage-pussy did its very best to show itself off to him — a clench brought the clitoris shooting outward and the muscular bottom of her pelvic floor racing upward, pursing out her energetic fillyhood. Those lips all parted for a moment, widening the still-minuscule tunnel just enough for a brief glimpse into the thrumming, hot-pink depths of her unexplored femininity… before it was occluded by a gush of steaming love-honey.

*Splat!*

A cupful of her hot fillycum splashed out from her depths, soaking her inner thighs and sending droplets scattering across the floor to soak into Caisson’s foreleg-fur.

“Celestia, I need this! I need you!” Cookie whined, lifting her hips up as high as she could. Caisson walked forward, his big hooves stepping easily around and over his much-smaller sister. Even though she was only a little small for a teenage-filly, Caisson barely had any trouble simply walking over and around her, his size meaning that he didn’t even really need to mount her.

*Thump* *Thump*

Caisson’s huge, unshorn-fetlock’d forehooves slammed down onto the haybale outside of his little sister’s. Hers were so small compared to his, so delicate. He lifted his big, strong neck up, before tilting his nose down to look at his filly. He let out a heavy blast of hot air from his nostrils, a wash of powerful stud-breath blasting against the nape of his sister’s neck and sending her mane flickering like white streamers.

“I fantasized… had always hoped…” Cookie shivered under him, looking straight up at him with those big, blue eyes, so full of want and need, “... hoped you would be my first, big bro.”

*Thwap!*
Caisson let his heavy stallionhood, still pulsing with almost unbearable tumescence, drop away from where it had been held rigidly against his barrel. It crashed down with a meaty, wet, slap against his little filly’s back, sending out a splattering splash of stallion-juices that had been accumulating on his dripping flare.

“Oh, Goddesses~! Cookie squeaked, her eyes flaring wider now that she could feel the sheer weight of his stallionhood on her.

His flare was nearly up at her withers, drenching the small furrow of her spine with the mixture of left-over ejaculate and drooling stud-honey.

Cookie! Caisson wanted to say her name, to call out his affection for his little sister, but the cry died on its way to his lips. Cookie!

Instead, a grunting growl rumbled up his throat. His consciousness was a faded spectre, a refugee bobbing in the churning waves of the tumultuous storm that was his mind. There was no gentleness, no hesitation in him anymore. Now he was a pure force. Love. Strength. Masculinity. Desire.

Instinct.

Caisson slid his rear hooves inside the stance of his sister’s hind legs deftly, quickly forcing her to spread them wider, lowering her posture and getting her to tilt her pelvis upward. She would need a wide-stance to accommodate what was coming.

Clattering backwards, Caisson grit his teeth, supporting most of his weight on his upper limbs as he started to position himself. Inch by inch his gargantuan stallionhood slid down his little sister’s back, leaving a wet trail of passion in its wake as it bumped along each vertebra.

“Ahhn!” Cookie whined, quivering as the log-like base of his stallionhood slid along the cleft of her buttcheeks, smushing her dock to one side.

With a level of coordination that could only be attributed to the assistance of millennia of moons of ingrained genetic routine, Caisson slid his hips in such a way as to pull his stallionhood off of Cookie’s back, and into prime position for sliding into his little sister — though his instinct-compelled mind considered her his soon to be his brood filly.

“Caisson!” Cookie groaned aloud, like a madmare, pressing her own dripping, gaping sex back at him.

The hot kiss of her filly-cunt against his huge flare jolted him like a sparking cattle prod. There was no deliberation, no confusion in his consciousness. His primitive, well-oiled stallion-hindbrain was in full control. It had flooded the remainder of his cognitive system with irresistible androgenic hormones that overrode each and every synapse, interrupting his every forming thought to interject with a singular, insistent compulsion.

‘FUCK HER.’

Everything else was burnt away in the crucible of this wanton desire, all that was left of Caisson the gentle-stallion were those emotions that would be useful to fulfilling this biological imperative: the love he had for his little sister, the cute-beauty that he had always seen in her, the desire to protect and guide her, to make her happy… those emotions were all enhanced and focused, sharpened to a point that would be used by the potent and ancient reproductive neural pathway prerogatives to achieve their objective.

Fill this filly with your big fat cock and pump her full of your seed.

Like a well-oiled machine, his hind-brain took in the sensation of fertile, wet, ripe filly-flesh against his flare and channeled the neural pulses through high-speed, logical circuitry — all quite independent of any conscious layer of thought — quickly arriving at the solution: thrust.

His powerful muscles tensed and fired, driving his legs forward in a tremendous burst of raw, animal power. Even if Caisson had been harnessed up to a three stone-boat-sleds filled with granite flagstones he would have made the whole chain jump with the force that he tried to penetrate into his little sister.

His flare smushed against her wet opening, an impossible fit even with careful consideration, let alone a wanton penetrating thrust. Their mutual, slick genital-wetness caused the force of the thrust to quickly slide off Cookie’s pussy and race up under her belly.

“Ahn!” Cookie squeaked aloud, as the huge flare-ridge bumped against her throbbing clitoris before grinding over teats and up the underside of her belly.

Caisson felt his cockhad missed its mark, confirmed by only the top of his mammoth stallionhood feeling the sensation of his sister’s body warmth. He growled in frustration as the sensitive head of his penis bounced up her underbarrel, finally stopping as his heavy hips bumped into Cookie’s and his huge, heavy balls swung up with leftover momentum to slap against the base of his cock.

“Ooomph!” Cookie grunted, taking the full impact of her brother’s thrust through her hipbones and bouncing forward against the hay bale she was propped against.

He had missed!

He fumed, blowing an angry snort out of his nostrils and stamping his foreleg. He didn’t even pause to consider that his cockhead was as far along as Cookie’s ribcage: if he had speared her with his goliath organ on that thrust, he might have knocked the wind out of her, literally.

Damnit!

“Whinny!”

He tried to curse aloud, but it instead came out as a terrifying growl-whinny, like an infantry-pony on the front line of a medieval charge. All his thoughts and actions were being converted into something more fierce, more primitive.

The rut was in full effect.

He quickly untensed his hind-legs, stumbling and shifting back to reposition himself, dragging the still-flared pink saucer of his cockhead up his little sister’s inner thighs. With a squeeze of his powerful pelvic floor muscles he tensed his stallionhood, making it rigidly lift for alignment once again.

He felt that brief, hot flash of teenage-snatch, and started to press forward. But he had already slid too high, instead pushing against the strange, bumpy ring-like tightness of a different orifice. The kiss of this tiny opening was very different from the other — it barely covered even a fraction of the surface area of his wide-flare… and there was no yielding in its clenched tightness.

“Whinny!” Now it was Cookie’s surprised nicker that shook the barn walls, directly communicating with him. “Stop!” Her left rear leg shot out in a violent, instinctive kick, glancing off the inside of his own rear leg.

Sorry!

“Snort! Huff!” His intended apology converted into a strange mixture of body movements, ear-flicks and breathing sounds.

They were communicating like horses now. Like tribes-ponies in the ancient plain-societies of Equestria, before their ancestors had become sophisticated enough to use words… and yet each little stamp and breath, each head-toss and ear flick, each clench, squeeze and drip… it was all as clear as if they were conversing on a debate-stage.

“C-caisson! Caisson! Please!” Cookie scream-whined, looking over her shoulder at him with an aching desperation in her eyes, like a drowning mare pleading for a life preserver. “I need you in me! Now!”

His little sister needed him to up his game… his broodfilly needed him. Now. Caisson quickly readjusted, bringing his cockhead down and back to the burning, pink gate of destiny, just as Cookie’s teen-pussy winked wide, her body doing its utmost to help her stallion, her stud enter her.

And it worked.

This time, the fat mound-tip of his flare embedded itself in his sister’s folds, not entering her, but capturing the furthest extremity of the overwhelmingly-large stallion-penis in its grasp…

And that meant that Caisson could push. The brain gave the go signal, powering up the bulky bio-mechanical machinery of the young stallion’s legs once again, this time with more caution and control.

He began to grind forward — not a wanton thrust like the first shocking touch against his sister’s dripping-pink flower... but a steady, increasing force like they might use when ratcheting out a tree-stump. Bit by bit he dialed up the intensity, transmitting all of the force of his fiercely strong legs, braced against the floor of the barn, up through his leg muscles, through his cock-base, up his shaft, and against that single point of contact where his huge flare mashed against his sister’s too-small filly-pussy.

“Ahn!” Cookie groaned, as she tried to brace herself on the hay bale with her locked forelegs against the tremendous force he was applying against her slender fillylips. “C-caisson!”

Her delicate folds were straining, stretching to accommodate his massive flare. Bit by bit, they spread outward, thinner and thinner, attempting to accommodate the bulk of Caisson’s cockhead. It seemed impossible — but mares are wondrous things, able to pass a foal’s head in childbirth… and this little filly’s body was ready to do equivalent miracles for her stud.

More! More!

Caisson’s stud-mind fumed, he did not want to be denied, not now! He pushed harder with his legs, driving his little sister’s body into the haybale pyramid, enough to make it tilt and sway dangerously.

But it was working.

Slowly, she was yielding… her tight young fillyhood expanding to meet the impossible demands of her huge mate. The wet heat of Cookie’s inner labia slid around the knobby edges of Caisson’s flare for the first time, enclosing him in a vise-like tightness of filly-flesh that sent a burst of sensational nerve stimulation into the big stallion’s pleasure centre.

*POP*

And then he was in.

All at once, hishuge head finally slid into Cookie’s pussy, her lips wrapping around the big flare and gripping the slightly narrower shaft underneath tightly. With the widest part of the top-half of his penis finally inside his sister, the powerful force both siblings were applying to try to cram as much cock into her small body quickly drove five inches into her body. Caisson’s cockhead was like a battering ram, bulldozing the squeezing confines of his little sister’s vaginal canal open as it plowed into her depths.

“MMmUGH! Holy buck!” Cookie choked out, like she’d been punched in the gut. “S-so big!”

But Caisson could hardly focus on her words — all he could do was bask in the sensation of perfect, wet, hot pink filly pussy welcoming his stallionhood. The swirling inner-folds of Cookie’s tight fillyhood clenched and whirled around his cock. It was so small and strong that the constriction was almost painful, and that was before his throbbing fullness would be squeezed by one of gripping, pussy-clenches that his sister’s body was firing off with regularity. But even that sensation was pure, white-hot pleasure.

And he wanted more of it.

Wanted more of himself inside her. He didn’t relent even an ounce of force from his driving rear-legs — never trying to withdraw even a fraction of an inch from the incestuous union they’d established — he’d go deeper, further, harder.

Slowly, the huge pink stallionhood sank into the squirming, squealing filly. Inch after inch of stud-meat grinding into her little body, each sopping bit causing obscene squelches and drips to emerge from Cookie’s plot.

“Ahn! AhhN!” Cookie’s head bent downward, her body tensing, focusing on accommodating her much larger older brother.

The shaft entering her now was starting to widen: Caisson’s girth started to grow terrifyingly as it approached his medial ring. He could feel his little sister’s clitoris grinding and bumping down the ventral side of his penis now, like it had before, except now he was stuffing her body with himself, which made the fat lovebud desperately strained against his unyielding rigidity. The bit-sized nub, stuffed with potent nerve endings, had been stretched into a crescent of throbbing, sensation-screaming pleasure… and it was booping and grating over his every vein and bump.

“F-f… Fu-ngh… AAAAAHH!” Cookie suddenly started to tremble and shake under him. Her tail lashed up striking his balls with whip-like, stinging force as she unintentionally tensed… then began to spasm.

He looked down at her, watching her head begin to thrash left and right, her muzzle biting into one of the binding straps of the haybale she was propped upon. And then her back started to arch and flex, whipping between convex and concave, wracked by powerful spasms… that went right through the powerful muscles of her filly-snatch.

She was orgasming.

Hard.

Milking waves of grasping, suckling muscles pulled and released the whole of his twelve-inches of embedded, gigantic, pink-hot, studly penis. Each thrum of her aching velvet-tunnel was a perfect milking movement, artfully designed by millions of years of evolution to coax him to run with her on those golden fields, to join her in orgasmic bliss and flood her with the rich, life-creating semen that her estrus demanded she acquire.

Her contractions spilled all the way to her strained lips, sending that stretched crescent of clitoral flesh shooting even further from its now well-exposed home. A massive gush of steaming filly-nectar spilled out from the clinched junction of pussy and cock, the nearly impervious seal sending the spray farther and harder, like holding a hoof over the edge of a garden hose. Hot lubricating honey shot down his shaft and splattered against his swinging ball-sack and rear legs — inviting him to release his own orgasmic flow, to pay her an equal measure of climactic payload.

But Caisson wasn’t even close yet.

His filly would have to endure. More sensation. More bucking. More cock.

Still, his mind was flooded by that deep hind-brain with positive reinforcement and another flood of androgen and magic-infused rut-hormones. A true stud satisfied all the brood mares of his harem — and feeling one of them cream herself was a sure indicator that he was the alpha-stud… and that her body wanted his foal, was ready for it. The surge of neurochemicals sparked through his chaotic mind, made even more potent by the natural earth-pony magic that made the race such prolific reproducers.

He thrust harder.

Deeper and deeper, through his little sister’s half-screaming, half-moaning, hip-shaking orgasm, pushing his pink horsecock into her tiny body. The girth of the shaft entering Cookie’s was now as wide as his flare had been, and only getting wider. He could only make progress as she relaxed during one of the tonic-clonic orgasmic convulsions that were still wracking her body underneath him…

But then she bumped up against his medial ring… and all progress stopped.

Three-quarters of the way down his shaft, just before the eighteen-inch mark, a thick band of throbbing flesh demarcated the difference between the terrifying size of his upper stallionhood… and the ridiculous expectations of his cock base.

Just like a massive redwood tree in the Everfree forest... supporting a huge, long structure meant that an expansive, girthy trunk was a necessity. The immense foundation was as thick as a mare’s leg. Caisson was as wide around at his enormous base as his cock was long — a mare looking to take his massive size ought to practice slotting a dinner-plate into herself first.

Just the medial ring was too much for Cookie: a mechanical stop on the flesh-column rearranging her insides. As the big ring ground against the teen’s tight snatch, pressing her forward, a deep, desperate moan rumbled out of the thrashing filly underneath him.

“Ohhhhnnngh…”

Caisson huffed, stamping hard into the haybale underneath his hoof in sheer frustration. He was not satisfied with this limit — his rut demanded that he overcome every obstacle, surmount the impossible, and completely stuff his brood-filly. But there was no more force that he could apply; already his powerful hind-legs were screaming with lactic acid, straining past the point at which injuries, strains and sprains could occur.

If pure, unending force wouldn’t work… then it would need to be a jerking thrust.

For the first time, Caisson began to pull backwards. Shifting his weight from his hinds to his forelegs, pushing back against the bales he and his sister were propped upon and achingly starting his withdrawal.

“Gaahhnnn, C-caiss!” Cookie’s whimpering shudder was a mixture of relief from the pressure, petulant disappointment at the removal of the cock she had desired so badly, and new pleasure as the huge flare began to plumb her depths, grinding against her twisting inner folds from a new direction.

As he slowly withdrew, his little sister’s pussy came with him — the filly-hood lips gripping and suckling as inch by inch his slid back out of her, dragging those dark and pink labia and her stretched clitoris as he went. The massive bulge in his little sister’s belly was slowly retreating as Caisson’s fat horsecock slipped out of her with a sucking, squelching sound from the immense suction of so much meat being displaced.

“CAISSON!” Cookie yelped in warning, as she had to grip the band of the haybale to prevent her whole body from being pulled backward, as if her teen-fillyhood was trailer hitch attaching her to a hydraulic hoist. Only because she was holding on with all her strength was he able to continue his slow, deliberate back-stroke.

But he wasn’t pulling all the way out.

Just enough to reset his purchase, to give his body enough leverage and run up to give a violent, sharp, bucking — THRUST!

With a mighty lunge and snort of air, he drove forward. The eight inches of slick, pink cockshaft he had just withdrawn plunged back into his little filly with knee-weakening force..

“GAH!” Cookie squeaked, as she was slammed back forward into the edge of the haybale.

The whole pyramid of stacked hay cubes jumped, shaking more violently than if Ponyville had received an earthquake: some of the top bales came crashing down tumbling and rolling off like straw-colored boulders, bursting into explosions of hay. Only by Celestia’s grace were neither of the incestuous siblings crushed by the heavy bales as they crashed all around them.

*POP*

But it worked.

Caisson’s girthy medial ring popped into Cookie’s tight teenage pussy, quickly introducing her body to the unbelievable girth of her older brother’s lower cock-base. Only some quirk of earth-pony reproduction magic could have permitted the impossible stretching needed as she sank lower onto the veined, pulsing base.

“Ahn! AhhN! AHN!” Cookie moaned out in little furtive clenches. Caisson could feel his little sister’s pert, athletic flanks pressing into the sides of his stallionhood’s base now: his girth was so wide that even her butt-cheeks were being touched by his impressive width.

Caisson felt a thump against the end of his flare — like he had slammed into an elastic, stretchy wall. There was no more depth for him to push into. He had reached his little sister’s womb, his huge knobby flare pounding against the gate to her deepest feminine core, smashing against the clenched portal of her cervix.

“Oh! OHHHH!” According to legends passed around the cafeteria table, some mares loved cervical contact, others didn’t. Judging by his little sister’s milking, climaxing clenches as he pressed against her deepest sexual gate, she was one of the former. “BC! BC!”

Splatters of filly-juice soaked down his tree-trunk base, gushing out with each new clenching spasm, running down his cock and waterfalling off of his low-hanging testicles in a waterfall of erotic lubrication.

Whinny!

A triumphal roar that would make even the fiercest of ancient, primordial horse-studs shiver in fear bellowed out from Caisson’s chest, as he straightened up and flexed his muscles in arrogant victory. I’ve stuffed my little brood filly!

The motion lifted his little sister up as his flexing cock rose up to his belly. Cookie’s hooves came up off the barn floor and the haybale — suspending her entirely in the air. She was being entirely lifted by his stallioncock.

Author's Note

Uh oh, she's going for a little ride.

Hold on tight Cookie!