Derrick's eyes were wide, showing whites all around, his ears laid flat and his nostrils flared. He felt his upper lip curl up, trapping more of the female scent and stimulating him to even higher lust. The raw power of his new animal instincts was scary, but since the stallion and Derrick both wanted to experience sex, he found himself giving in. Besides, the huge black mare was practically raping him - it was probably rude for him to refuse her.
Making the decision was easier than accomplishing the job - he was still not really used to hooves, and having two legs was not quite the right design. Luckily, the mare was obviously experienced and shifted her rump to help him find her opening. Derrick felt frustration as he missed once, twice, but finally scored a direct hit on the third try. By this time, his enormous equine shaft was rigid and stallion's need and blind lust were almost all that he could think of.
Pushing forward, Derrick fell over the mare's back, instinctively gripping her sides with his arms as he drove deeper. Her inner muscles clamped around him, and he whinnied in pleasure as he started to pull back for his first real thrust. A wave of pleasure, pressure, and a curious sense of increased mass rippled through his body, and he heaved over the mare's back, each powerful thrust bottoming out. By the fourth thrust, Derrick had no conscious thoughts at all beyond the totally equine need to mate.
Release came suddenly and all too soon - he shuddered and they both whinnied as his massive testicles release what must be a quart of semen, blasting the thick seed inside the mare like a firehose. He thrust in twice more, then slid off her back to stand panting behind. The amre shivered, and then turned to nuzzle him. "I figured it would only take once. You're a fine stallion now, loverboy!"
As Derrick's heart slowed and the mind-numbing pleasure of orgasm faded, he realized he was resting his weight on his hands. No, his forehooves. He had fully-formed equine legs instead of arms. That should have been a shock, but Derrick just accepted it. He did not have to examine anything else to know he was fully transformed into a Shire stallion. It was something he knew instinctively. His entire human life and memories were still present, but now those thoughts and that existence seemed irrelevant. He was a horse.