As I pondered what the beautiful Fillion had said, I felt my cock harden and grow between my hind legs. It hung noticeably underneath my equine belly, throbbing, desperately needing attention. It's true that I felt no compulsion to mount or otherwise fondle the unicorn, but something about my new body seemed... off. I felt somehow disconnected from my lower body, as if my head, shoulders, arms, and torso were simply attached to the body of a beast different from myself, as if my lower body was not under my control. Suddenly, I felt myself walking towards Fillion, though I had not ordered my legs to move.
"That's the spirit", said Fillion, ever presenting me with his plump, smooth, unicorn rump.
"But I... this isn't..." I tried to explain myself to Fillion, but was too distracted by the feeling of mounting the unicorn against my will and the sensation of my heavy cock sliding into the unicorn's warm anus. My lower body began to thrust. Though I wasn't controlling my lower body's movement, I felt everything that it did. I was simply along for the ride, the top half of a human perched atop the shoulders of a lustful stallion.
I couldn't deny that I liked how it felt to fuck the unicorn. His anus was just the right fit for my large cock, and I felt my lust grow hotter with each thrust. Realizing there was nothing I could do to stop my lower body from this sexual act, I decided to make the most of the situation. I massaged my chest and tweaked my nipples, amplifying the sexual sensations I felt all throughout my body. It wasn't long before I reached a climax and released a heavy load of warm cum in the unicorn's rear.
Though I had finished, my lower body remained mounted atop Fillion's back for a few seconds more. Try as I might to pull myself off of his back, there was absolutely nothing I could do to move my lower body. After a few moments, my lower body finally backed off the unicorn's back and stepped onto the floor.
"Fillion!" I shouted. "What did you do to me? My lower body--it's not me!"
The unicorn sighed, "I'm afraid I couldn't help myself. Seeing how beautiful you are, I didn't want to risk losing your affections, so I... made some adjustments to your body. I made it so that you CAN control it completely most of the time, but as soon as I'm in the mood, your lower body reacts to my desires so that I can be satisfied."
"You mean I'm like some kind of on-demand fuck buddy?" I said, staring at my lower body in horror. I had no idea being a centaur would have gotten me into such a predicament.
"If you don't like it," said Fillion, "I can make you once again vulnerable to Runt's magic."
"No!" I said. "I don't want anything more to do with him." As I spoke, I felt sensation return to my lower body and I practiced trotting in place. "I guess... I could get used to this."