As Dave continued to slam himself inside of Mary, he noticed something odd. Mary was whinnying but not as much as before. What's wrong with her he thought. Then it hit him: his penis wasn't giving her the same pleasure as he was getting. Her vaginal walls were way too big for his puny human cock. Dave didn't care. He was going to finish, and that was that... until he felt something odd.
While Dave continued to thrust, he felt a strange sensation in his penis. It felt like it was growing and gaining more mass with each stroke. He could feel his balls begin to grow with his cock, as they went from human to plum size. As this was happening, Dave began to feel himself begin to fill up Mary. She was whinnying louder again, and Dave could feel her big body tremble underneath him. He felt the pleasure beginning to build and build. He snorted and became startled as caught himself whinny. "What the--??"
Instantly, Dave could feel himself changing. His once, tight butt began to bulge and beef up, as he felt something begin to sprout from the rim of his wast. He moaned as he felt the beginnings of a tail begin to poke through. In addition, he felt his ears become pointy as, he could feel his face begin to push out.
Both Dave and Mary were ecstatic now. Mary was panting now, as she could feel Dave's member inside her lengthen and grow thicker. Not only that, but Dave was beginning to grow heavier. She could feel his belly begin to barrel out and push against her's, as she felt his smooth skin begin to sprout fur. His cock was getting even bigger inside her. It felt so big and strong, as Dave continued to pound it into her walls. They both felt the pleasure building and building until they both climaxed. Dave felt himself release a big load into Mary, and she nearly screamed at the feeling of bursting with his load.
Dave collapsed next to Mary panting. Within seconds both were fast asleep, exhausted from the experience.