Fillion and I had spent quite a while alone together and he had since used his powers of persuasion to make me fuck him involuntarily twice more since I discovered I only sometimes had control over my lower equine half. I'd be lying if I said that I hadn't grown to enjoy sex with the unicorn despite only having been a centaur for a few hours. I thought back to my life as a human and convinced myself that I'd be happier as a centaur, even if it meant living at the whims of a horny unicorn.
Our fun was cut short, however, when Runt came looking for us and caught me mounted atop Fillion, thrusting my equine cock uncontrollably into his unicorn anus.
"I have to say, I'm a little hurt," said Runt, making light of his true feelings of displeasure. "I didn't think you'd be the type to stray."
"I'm, uh, I'm sorry, Runt, the unicorn... uh, I lost control of my lower half, I..." I tried desperately to find an excuse. I remained ever mounted atop the unicorn's back, bucking my cock into his anus. Despite Runt's presence, Fillion still seemed to be enjoying himself.
"Remember, kid, Runt can't control you anymore. My magic protects you from his... ooh! Oh, yeah, just like that..." Fillion moaned as my lower half thrust more forcefully into the unicorn.
"That so?" asked Runt. "We'll see about that. If you're gonna take my gift for granted, how about I turn you into just a regular horse?" Runt jumped onto my thrusting equine back and grabbed at my arms and face.
"Hey!" I screamed, "What are you--oof! What are you doing?!"
"I'm--irk! I'm--turning--you--into--a--horse!" Runt's spoke in staccato as he bounced up and down on my thrusting back.
Fillion laughed as I continued to fuck him. "You can't! Don't you get it, Runt? He's safe with me--you'll never be able to transform him again!"
Runt jumped from my back. "No fair!" He stood and watched me as Fillion and I continued to mate. "Well, what about THIS?" Runt ducked between my hind legs and grabbed at my penis, which was so long that not all of it could fit into Fillion's butthole. Runt massaged the base of my penis and I felt it begin to change.
My penis slid out from Fillion's butt and shrunk agains my equine belly. I soon felt control return to my lower half and I quickly jumped off of Fillion in shock. "NO!" I screamed, "NO, DON'T take my penis again, PLEASE!"
Runt laughed, "Oh, I'm not after your cock this time. I'm just here for some fun." Runt stood up and ran behind me. "Tell me how THIS feels, ya fillie!"
I felt a completely foreign sensation in my rear end, one that filled me with unrivaled ecstasy, but one foreign nonetheless, "OH! OOOH! What the... Runt, what did you DO?"
"You're stallion no more, kiddo! Nope, below the waist, you're now a fully functioning MARE! Ha!"
I quickly put my hands to my face and noticed that it felt the same as it ever had. I looked down and felt my pecs and rubbed my hands across my svelte stomach. I was still me from the waist up. Male. But the feelings in my rear end from Runt fucking my equine pussy were proof enough that I was, in fact, a mare below the waist. A centaur still, but a human male from the waist up and a mare from the waist down.
I tried to ignore the rapturous feelings coming from my rear and begged Fillion to explain what Runt was doing to me. "I thought you said he couldn't change me anymore! What the hell is this? I've got a HORSE'S vagina!"
Fillion appeared shocked. "I'm sorry, I... in making your lower half obedient to my desires, I must have forgone making it immune to Runt's magic..."
"Well, isn't there anything... OOOH!" I moaned in pleasure to the feelings of Runt fucking my horse pussy, "Can't you... change me back... now?"
Fillion shook his head, "I... I don't think so, at least not while he's... inside you. I'm... so sorry..."
Runt laughed "Hey, enough of your chit-chat! Let's have some more fun! Let's turn you into..."