Mike suddenly felt nervous, though the mare's scent was strong and inviting. Did he really want to do this? He shifted position without thinking, his horse cock swinging stiffly under his black-furred belly. The mare leaned forward, dropping her head, and urinated on the stall floor. She was in heat, she wanted him, needed him. Mike breathed in the smell of her pee, which exited him even more and fired up powerful instincts. He shifted closer, rubbing his equine chest against the mare's rump. She whinnied and bumped back, her black opening clenching and unclenching in initiation.
Liam, the stable boy, watched him with a smile. Maybe it was normal for centaurs to do this. Not that Mike really cared any more. He had to mount the mare - any ability to choose otherwise had been overridden by the need of a Shire stallion. He shuffled back, prancing and shivering, and then lunged forward, pushing up over the mare's rump. Sliding forward, he gripped her sides with his forelegs and leaned forward to hug her neck with his arms. But he had to release and slide back for he missed her opening, and then missed it again. Grunting in frustration, Mike slid back a third time, only to feel hands on his cock. He twisted back to see Liam grinning at him, then instinctively thrust again, this time guided into the mare with the stable boy's help.
Pleasure! Mike's mind was overwhelmed by the intense sexual sensations and base instincts as his huge shaft drove deep. The mare whinnied and he grunted, rocking back and then driving in again. The mating was brutal and totally without intelligent thought. He shuddered and thrust a third time, then a fourth. Pressure built in his testicles and the base of his shaft, and he only managed two more thrusts before he exploded into the mare, throwing his head back and flailing his arms as he flooded her with his seed. He rocked back and forth a few more times, sending much smaller spurts into her, and then slid off her back feeling flushed and incredibly satisfied. It took a few minutes for his thoughts to clear, during which time he stood next to the mare and rubbed against her. Both he and the mare were lathered from their exertions, but the smell was wonderful, not bad. He was aware of his entire body, and it was all throbbing with power and maleness and mass. Mike had never imagined it was possible to feel this fantastic. He had masturbated as a human before, but never actually had sex with a girl. The difference between sensations was staggering. He was pretty sure it was more than just size - the Shire stallion's instincts and needs combined with the mare's clamping pressure to be an overpowering sense of fulfillment and pleasure.
"I'm guessing you liked that?" Liam leaned against the stall gate and grinned. "I know a breeder in another town who has a whole herd of Shire mares and fillies. He's been looking for a champion sire for a while. I could set you up there as a Shire stallion. The feelings are even more intense for a full horse."