You spend the rest of the day rejecting the lovesick horse and donkey, trying to make up your mind. If you have sex with the big draft stallion, you'll have a horse foal. If you get mounted by the jackass, you'll have a mule. Of course, you could let them both screw you and see who's sperm wins out.
The boy takes you all in to the stable as the sun goes down, and you are ready for a quiet night alone in your stall. But a few hours later, there is movement outside and your master shows up again. That seems unusual, especially when you realize he is naked!
He comes into your stall and shuts the gate, then moves next to you, patting you and telling you what a good girl you are. Looking back, you see that he has a raging hard-on. You are shocked at first, then think the kid is being pretty silly. After all, he is hung pretty well for a human, but he'd barely get inside your horse cunt.
He moves beside your rump and uses his fingers to gently massage your opening. Still in heat, it doesn't take much to get you slick and hot, longing for the attention of a male. His hand slides in, and then part of his arm as he uses his fist to pleasure you. The human arm and hand are pretty close to a stallion;s cock in size, and you feel yourself clamping down around them. When he finally pulls out of you with loud sucking noise, you are already shivering in pleasure.
He leaves for a moment, and you are terribly disappointed. But then he returns with a wooden crate that he sets behind you. Standing on it, he positions himself and presses into your hot female slit. As you feared, he can't penetrate far enough to do much for you. After getting excited by the foreplay, you feel a growing frustration and tremendous desire to be filled with a huge equine cock. You don't care if it's the stallion's or the jackass's.
Then you realize that the boy thrusting into you is actually managing to get a little deeper. Encouraged, you try working his enlarged dick as you did his arm, and are rewarded by even more penetration. Your master is gasping now, making slower, longer thrusts. Much longer. You feel him almost to the point his arm had been, and his shaft feels almost as thick!
He shifts behind you, almost falling off the box. Then he kicks it away in his passion. Instead of pulling out of you, he only drops down a few inches! The building pleasure and sexual instincts make it hard to focus on anything else, but you manage to look back at your human lover.
Your not-quite-human lover, that is. The boy has gained a good two feet in height, and you can see hooves on the stall floor behind you, not human feet. It's hard to be sure in the darkness, but his face looks pushed out, and his blond hair is a lot darker.
By now, you can feel him deep inside you, far deeper than any human could manage. Although this is your first time, you know a fully equine cock is sliding back and forth inside you. The flat tip is flaring, bringing you to full climax, and then he erupts with hot liquid inside you. Another pulse, then another. You both shudder in pleasure as he empties the massive black balls swinging behind his massive black cock.
He lays over your back, panting and sweating as you continue to massage his erect organ with your own internal muscles. Now that the climax is over, you can look back. He is definitely more animal than human now, and you realize that he is changing a little more with each squeeze of his cock.
You are changing him! Curious and excited, you try imagining him as a Clydesdale. As you continue to work him, you see the darkening hair on his head get lighter again, and he seems to get bigger. You can even decide what to make him! But do you want to make him a horse? Who will take care of you? Can you make him human again anyway?
There isn't much time to think about it - he is changing faster now. You clear your head and focus hard on turning the boy into: