You struggle against the urge to buck your hips so as not to hurt your girlfriend as she gurgles beneath you, your equine rod crammed into her mouth and partway down her throat. You feel her hands tugging at your huge balls desperate for your cum, but she soon lets go as you hear a soft thud followed by a moan as she continues to slobber on your cock.
Though all you can see of her is her increasingly equine backside below you, you swear you can feel your girlfriend's mouth getting deeper as she swallows more and more of your cock. Overwhelmed by your new instincts you think little of it and soon reach your limit, jetting thick ropes of cum directly down her throat as you watch a tail shoot out from above her now fully hair covered ass.
As post-nut clarity briefly clears your arousal you suddenly realise that a centaur's mouth wouldn't be getting deeper. Alarmed you step back, bringing your girlfriend into view as she gasps for air in a heap on the ground, as she sits up the full extent of her transformation is revealed.
Your girlfriend's body is now almost fully covered in blonde hairs, her arms now ending in a pair of hooves where her hands had been, trapping her on all fours as she struggles upright onto what are now her four legs. Most alarming of all her face has stretched forward into the beginnings of a horse's muzzle, her features a strange blend of woman and horse.
At a loss for words you realize that soon your girlfriend will be nothing by a horse, but despite yourself all you can think off is putting a foal in her, your instincts demanding you breed. As you watch a pair of equine ears begin to poke out of her hair you gaze finally meets hers, a look of understanding passing between you as she turns and her new tail flips to the side revealing a vagina more horse than woman.