As soon as he pressed the button, Jeff could tell that the program was starting to change him. His whole body felt warm and tingly, from the top of his head to the tips of his toes. His dick stirred, and began to inch out of his specially designed underwear. He quickly removed all of his clothes, and sat down on the bed naked. He knew was going to get bigger, and he didn't want to suffer in tight clothes. He was also very excited about the change, and wanted to see everything as his body transformed.
After he removed his pants, he saw that his legs had already changed. His thighs were thicker, and continued to expand before his eyes. His calves and feet stretched longer as he watched. His dick ached and throbbed, and when he touched it to guide it out of his way, he suddenly had an orgasm, releasing a massive amount of spunk. The orgasm seemed to last forever, and although it was bliss, he didn't think it would ever end. His vision swam. When he refocused, he was amazed at the amount of seed he had released. It was dripping from the ceiling down his wall, and appeared to be enough to fill a bucket.
I guess I should have thought about that before I started, he thought. I need to clean that up after I'm finished.
He looked down to his thighs, and saw that they were continuing to grow, getting longer and thicker. He noticed that his legs were also getting hairier, with thin grey hairs stretching across his thighs and down his calves. I guess I'm getting manlier; maybe I have more testosterone or something? he thought. He looked at his arms, and saw that they were hairier, too. He was a little surprised that although his arms were longer, they didn't have quite the same level of muscle development as his legs. He flexed his arm, and his bicep was still small, almost the same size as he was before. His chest, however, was developing quite nicely, becoming a fine set of pecs. He felt them, and was little surprised to find how sensitive they felt, almost sensual. His cock responded almost immediately, and rose to attention. He grabbed it with his hands, and was relieved to see that although it was still large, he could feel it shrinking in his hands. The feeling of his hands sliding down the shaft of his cock as it shrank was all the stimulation he needed, and he had another massive orgasm, shooting even more cum from his dick. The pleasure was so intense, he almost blacked out.
When his orgasm finally ended, he saw that the amount of cum was less than before, although it was still an impressive amount. Must be because my balls are smaller, he thought. It was a little concerning, but not unexpected. What really bothered him was the hair. It was thicker now, entirely covering his thighs, arms and chest. It stretched over his thighs, and past his groin and ass onto his abdomen and back. He noticed that the hair was a pale grey color, with some small patches of white hair on his upper thighs and hips.
Something's wrong, he thought, This hair is weird. It's almost like fur. And those spots - they look like something you might see on a animal.
Suddenly, he felt a sharp pain in his feet. He began to rub them, for comfort. As he watched they began to rapidly lengthen while staying the same width. The bones ached, and felt like they were grinding together, fusing. His third toe grew rapidly, thickening and pushing his other toes to the side as they shrank and receded. His nail grew, becoming thick and black, stretching below his remaining toe, like a hoof.
Oh, no, he thought. I'm becoming a horse hybrid just like Elizabeth. She must have done something to the program. He tried to stand, but felt wobbly on his new feet, and fell back on the bed. His weight dislodged a pillow, that brushed against his still sensitive cock, and he was again lost in the pleasure of another intense orgasm.
When he came around this time, his face felt different. He ran his hands along his face, and felt his wide nose with enlarged nostrils stretching to the tip of his mouth. A long thick tongue snaked out and ran along his lips experimentally. His face felt soft and furry. He reached up and touched his ears. They were much higher up on his head than he expected, and were longer, thinner and sharply pointed at the tip. His hand brushed against his hair, and he pulled some of it in front of his face. It was very thick, and white, like the spots on his leg. As he held it in his hand, it continued to grow getting longer and thicker, stretching down his back. When it reached his ass, he felt a slight pinch, as a small mass poked out of his lower back, and began to grow longer. As it grew, long white hair began to grow off of it, getting thicker and longer with each burst of growth.
His dick ached again, more excited than it had ever felt before. It begged for another release. He reached down and began to stroke it, trying to help it along. He was surprised to find that it felt small in his hands now, and was continuing to shrink as he massaged it. His balls looked about the same size, but his scrotum felt tight, like the skin was shrinking around them, and pulling them into his body. He continued to rub his cock as it shrank until it was nothing more than a small nub, then he began to massage the area around his scrotum ferociously, noting with some fear that his balls were now inside an open slit in his groin. Finally, after massaging the slit seemingly endlessly, he was rocked by the most powerful orgasm of his life, and rolled back onto the bed in ecstasy, curling his toes and hands reflexively.
When he was finished, he rose from the bed shakily, and walked to the mirror to examine himself. He was tall, not quite as tall as Elizabeth, but much taller than he was before. His fur had a pale grey color, and his mane was a pale white. A few white spots dotted his hips and upper thighs. Around his wrists and calves the hair was white and much longer, hanging in long strands off his appendages. Thankfully, he still had somewhat normal hands, with five fingers, but they ended in hard black nails, as thick as hooves. He had an athletic frame, with thin arms, a toned, tight stomach, and thick muscular thighs. He had a very feminine shape, with a very thin waist, and a nice hourglass figure. He turned and lifted his tail to examine his ass. It was heart-shaped, very large, but fit and firm. He had a large chest, with pert tiny nipples. And of, course, he saw that his groin was now a horse's vagina. His worst fears were realized. He was now a anthromorphic horse-girl, just like his sister and Elizabeth.
A voice coming from behind him startled him from his examination. "Tsk, tsk, " it said.
Jeff turned and saw Elizabeth standing in the doorway of his room, leaning against the doorframe, with her hands crossed. The door was intact, but he could see that the lock had been broken as it was forced open.
Elizabeth smiled at him and said, "You should have listened to me. I warned you not use that program."