There was an overpowering sensation of pleasure and fulfillment as I ejaculated into the mare... and then nothing. I seemed to be floating in warm fluid, total blackness, with a dull, rythmic pounding I both heard and felt. This continued for a long time, though the space around me appeared to close in. Then there was sudden pressure all around me, and I was pushed out into bright light! For a very short time, I was looking up at the back end of a mare, her opening wide and slick, and then the mare shrank away to become a familiar-looking young human male. Runt?
He stood over me, grinning. "Certainly my most ambitious effort! How do you feel, my new son?"
Son? Then I sat up suddenly, seeing the afterbirth still covering my body. My human-looking, but also very young, body. I was not an infant but couldn't be more than seven or eight. I felt normal, though, not weak, and was able to stand easily. The afterbirth slid off, leaving a slick coating. "What did you do?" My voice was a child's as well."
"Took what was left of your human soul, which wasn't much, and created a new body for it. Never tries it before, but you seem to have come out all right." Runt walked around me, a head taller now. "Those elves did a number on you, making you think you wanted to be a horse forever. Not that you didn't help. You may want to end up a horse again, but this time I took the initiative and saved your sorry ass."
I looked down to get a better look at myself. Something didn't seem quite right, though I did appreciate the uncut penis that was good sized even in pre-pubescence. Then I realized what I was missing. "My navel! I don't have a bellybutton!"
Runt grinned. "Nope. Despite the mare's womb, you are actually demon-born now. Literally my son."
I twisted around and sighed when I saw my tail and reached up to feel my horns. "So, I am a demon now?"
"Not quite." Runt used his hands to scrape off the muck on my skin. "Maybe a demon in training, or a baby imp. You could even end up human again, but I doubt that will have much appeal." He made a gesture and a huge mirror appeared in front of us. I stared into it. There was no doubt who my daddy was. I was obviously a younger version of Runt, a beautiful child who glistened with the sheen of residual afterbirth.
"Can we go back to my apartment now? I think I need a break from magic and strange creatures."
That got a laugh from Runt. "Says the kid who is a strange magic-capable creature."