You look at Steph with lust, his bovine/equine donut puckering with lust as well. Your cock and balls slowly shrink; still well above average for a stallion of your stature but more manageable. You use your giant tongue to sample the goods and he shudders with pleasure. You whinny loudly and mount her; instant ecstasy as if her ass was made for your cock. In, Out, In, Out; he storts with lust at every plunge.
*SNAP!* You hear your back crack loudly and begin to ache, but you're so deep in your rut that you keep drilling him. *SNAP!* goes your back again, the ache getting stronger. You don't notice yet, but your back is getting longer slowly but surely. *SNAP! SNAP! SNAP!* You suddenly stumble for a second as your center of balance shifts and rest your weight on Steph. He stumbles a little as well from your weight, but moos for you to keep thrusting; your cock still inside him.
You reposition and begin thrusting again, *SNAP!* you loose your balance and put your left hoof on the bed. But you're still horny and go to thrust again. Stuck. Your underside is suddenly really stuck to his back. He looks up at you in confusion, but as he does that his head touches your chest and is suddenly stuck as well. *SNAP! SNAP! SNAP!* Your neck lengthens and you look in the mirror, seeing two bumps on either side.
His tail suddenly lifts up and absorbs into your body; he struggles in fear and shock. Your tail starts to extend out, turning into an old school unicorn tail. *SNAP!* Your neck stretches again and the bumps grow. His horns submerging into you: a unicorn horn painfully emerging from your forehead. He looks in the mirror at your in pure terror as his legs start to shrink into his body. *SNAP!* You can see the bumps are becoming arms; You Are Becoming A Taur.
Your cock flares as you cum buckets into him, his stomach buldging full. His cock darkens to match yours as his rear begins to absorb into you; he cries out in a long fearful moo. You now have his udder propped up on your new cock, well technically your udder now.
*SNAP! SNAP!* Your neck turns into a torso, neck, and arms. He lets out one final cry as he becomes fully absorbed, giving spots to your snow white coat. You rear back and cum again, coating the mirror in Jizz.