As her mum busied herself organizing a willing farm to come collect Dan, Kat's guilt grew, she couldn't leave her brother alone on some farm, it was just meant to be a joke, what had she done? This wasn't right, she would have to go with him, but she couldn't as a human, as the idea formed she grew more determined, she had to do it.
Dan looked up as Kat snuck out the back door and approached him putting a finger to her lips, she untied him, he was now a full horse, the spell's effects had continued to alter his body.
Now out of earshot from their parents Kat turned to Dan and told him she would make this right, though she couldn't turn him back she could at least make things even. Shaking at the thought of what she was about to do, she cast the spell, and after a few moments the transformation began.
It began with a sudden pressure in her rear, moaning Kat felt her ass begin to swell as it grew in size, she should have removed her clothes first she thought as the pressure in her pants grew. Soon the pressure became too much and her tight pants exploded off as her soon to be equine ass broke free.
Dan looked on, unable to contain his lust his huge new member began to unsheathe, this may be his sister but what was happening before him was undeniably erotic. As Kat's ass continued to swell her legs joined in, thighs balooning out to be able to support her growing mass. She kicked her shoes off as her feet contorted into hooves.
Kat lost balanced, unused to her new hooves she fell forward, startled she felt hot breath on her ass, she had fallen right in front of Dan's face. She knew she should be embarrassed but all she felt was a new alien lust, something wasn't right.
Kat had made a mistake in the spell, she wasn't becoming just a mare, she was going into heat. She let out a long, drawn out moan as her vagina shifted position, moving up her ass to it's rightful place under her tailbone, allowing easier access for a stallion. She felt a twinge in her spine and a tail erupted from it's base, rapidly growing to full size and becoming coated in blonde hairs.
Next Kat's vagina began to pulsate, her once small petite pussy lips began to darken, swelling to become those of a mare, she felt inside her vagina stretch and grow to accomadate an equine member. Her sex was now no longer distinguisable from that of a common farm animal.
Seeing what was happening Dan could barely contain his new equine instincts, sister or not, a mare was forming before his eyes. Kat let out another gasp as her body continued to change, next her arms rapidly shifted into forelegs and her hands melded into hooves. Her torso began rapidly expanding, her top and bra lasting mere moments before ripping and falling away. Blonde hairs began coating her entire body.
Now Naked, Kat shivered as her breasts pulsated, melding together and darking like her vagina. They shifted down the length of her body and settled into position between her legs, now teets for her future foals. Foals, as the idea formed in her head it grew increasingly appealing, her equine instincts were taking over.
Next Kat's neck thickened and lengthened, leaving her as a full horse save for her human head, she turned to look at Dan and no longer able to contain herself she shifted her tail to the side, giving him full view of her equine pussy. "Mount me damn it! She gasped, make me your mare, put a foal in me!"
As the words reached his ears, what little of his inhibition crumbled and he reared up, landing on Kat's back, now able to support his weight. He prodded around with his huge penis, before finding his mark and plunging inside. This mare would be his.
As their mother got off the phone with yet another unwilling farm she hoped her husband would have more luck, he had left to go ask a friend of his who was a farmer. Suddenly she heard her Daughter's vile mating plea. What had the fool done!
Running outside she was met with the sight of her now equine children mating, Dan's huge member plunging in and out of Kat's dripping folds. "You idiots! What are you doing?" Kat turned and looked at her, "mum this is amazing, you have to try it", she moaned as the last of her human features faded away, her head now also that of a horse.
Their mother looked on in aroused horror as Dan's pace quickened, suddenly he thrust deeper than before and let out a satisfied neigh as his penis pulsated, beginning to pump Kat full of his virile seed. Their minds began to devolve as they were overwhelmed by the pulsating bliss, they would soon be horses in mind as well as body.
As their mating concluded the last vestiages of humanity drifted away. She was going to have so many foals, thought Kat, just before her human mind vanished.
Now only two horses stood in the yard. One of whom was clearly impregnated, the mare that was once Kat grazing on the grass, cum dribbling from her folds and down her long legs. What could their mother do now?