You just groaned as your uncle hauled you to your feet and led you to the birthing stall. Soon enough you were lying on your side in the deep-banked straw on the birth-stall. Your uncle fixed you up a nest of blankets and pillows before bringing in your clothes and books. He kept your phone, however---'Can't have you go twittering pictures of that belly and getting the cops involved! This is on the off chance that it actually works' you tugged urgently on his sleeve as you tried to straighten back up.
"What did you mean, 'till my body changes and I've gotten pregnant and had my first foal'? I can't foal; I'm a human!"
Your uncle just laughed a little.
"You sure know how to pick 'em," he called across to Zeus who hadn't left his stall yet, before clapping you on the shoulder.
"Look, son, centaurs can breed with humans just like stallions do with mares. Yes, even men. No, I don't know how it happens; jus go with it and it'll be over with soon enough. As fer your body changin you're his mare now. You'll literally have a mare's Vagina, anus, and reproductive system. Heck since you're his mate you'll probably get a few horse instincts like a mare's and even go into heat once a month for about 8 days till you're nice and big with foal. Then you'll just want to be fucked for pleasure."
You looked at your uncle slightly horrified and embarrased but couldn't bring yourself to do much more as he had started laughing once he saw your embarrassment at your arousal
"Judging by that slick runnin' down your legs I can tell you've already got all the right parts for breeding and foaling now." Mortified you just lay there even as your arousal grew.
"Your diet might change a wee bit too, along with you possibly growing a tail and your ears changin' to horse ears. There really shouldn't be any other changes than that. Yer one of the lucky ones 'cuz most men who get fucked just get pregnant and foal without any changes. After foalin' they usually forget completely bout what happened and their lives go back to normal. In your case the changes will be permanent cuz you're his mate. So come fall you'll have to find a way to manage your heats as you won't be around Z to get fucked. Less you can convince yer parents to transfer ya to school here and I'll make sure ya stay with Z here." With that your uncle left and Zeus came over smirking.
"Ready for more?" You nodded eagerly once again going down along all fours which just felt natural, presenting your ass and brand new vagina for him to use and pleasure you. You were getting wetter and wetter from anticipation the closer he came, soon enough he was once more on top of you and this time he was lining up his cock with your new vagina. Suddenly he thrust inside you with no warning, a thick, horse cock slammed deep into your newly made virgin pussy before hilting out inside you it only hurt for a second when he popped your cherry. You noticed that being fucked was now pleasurable and you couldn't even tell that his cock was in you unlike before when it was extremely painful, this was MUCH better. Slowly he started pumping in and out of you faster and faster. You whinny from the pleasure of being filled to the brim and your first orgasam as Zeus's member finally thickens. He grips your hips and rams his cock even deeper into you, further into your vagina before with a low moan he begins releasing gallons upon gallons of his seed deep into you once more. The seed was hot and thick, and instantly made your gut round out with the flow even though he was fucking your pussy. The urge to mate was stronger than ever as torrents of cum dribbled into your pussy, Zeus only grunted as he continued to expel his seed into you. Your stomach had become quite large once again at this point but you felt no discomfort or nausea. Pure simple pleasure, and as he unloaded the last of his seed into you you came once again with an extremely loud whinny. Soon enough he was finished fucking you and once again flaccid, you felt him slide out of you so you turned around to ask him for more. You wanted to ensure the best chance of having a foal, and to do that you needed your mate to fuck you more to ensure your egg (s) got fertilized.
Zeus simply laughed upon seeing your crestfallen expression. "Just a few moments ago you were protesting against me fucking you and now you can't get enough? You're such a slut. Don't worry though you'll be pregnant soon enough if you aren't already. I stopped for now as one of your uncle's hired hands is approaching the barn and we don't need him to find out anything."