"I want to be your identical twin, a goat-demon."
Runt's eyes flashed and he bleated as he suddenly thrust his goat-cock into me. I gasped in pain and shock, for it was not only bigger than I thought, but so hot I though I might get burned. His tail whipped around us, and his arms squeezed my chest tightly. His hips bucked as he began to hump me in earnest, each thrust deeper, hotter.
But the more he mounted me, the better it got. It was still hot and deep, but the pain lessened and I got a more pleasurable sensation. The heat spread out quickly, becoming an intense warmth all through me. I felt throbbing in my head, and prickling all over my skin. I became aware of a second bleating joining the animal sounds that Runt was making, and realized that they were coming from me. It was hard to think of anything beyond the huge member pounding into me, or the heavy animal musk that filled my nostrils.
I knew things were changing - my face protruded out in a partial muzzle, my feet were numb and clacked on the floor, and my senses were becoming far more intense. I could hear, smell, taste, see everything with incredible detail. And I could feel my whole body throbbing with pleasure as Runt continued to thrust, his massive ballsack thudding softly against my own testicles. His thrusts got faster, deeper, harder, and all I could think of was that massive goat cock. And then he hugged me tightly as he released a flood of thick, warm liquid inside me. We both bleated, repeating twice as he came again, and then emptied with a third massive gush.
We remained connected, his arms around my chest and his cock deep inside me, for a few minutes. Then he pulled out slowly. I could feel liquid dribbling out of my anus, with some squirts caused by the pressure of semen.
Runt changed back to the beautiful boy as I turned to face him. He grinned. "Gods, you are ugly!" Moving aside, he pointed to the mirror. Sure enough, I was the black goat-demon. Black all over, with yellow goat's eyes, curling horns, and three-fingered hoof-hands. I ran my fingers over my shaggy coat, amazed at the sensations.