Jeff knew he could never go back to being a human, so he decided he might as well have some fun. He thought "f/Hi/Hi" and felt himself changing.
Fang's penis slowly slid up into his body, becoming a nice, firm dog pussy. His balls, bouncing to the neighbor's dog's thrusts, slid through his new pussy with a pop and a yelp. Jeff knew he could now control this new aspect of Fang. Jeff quickly made Fang in heat, so the neighbor's dog would hone in on the more interesting hole.
The neighbor's dog was quite enjoying himself pounding Fang's anus, but the smells coming from the new vagina were too powerful.
The dog yanked his knot from Fang's ass, sending a torrent of cum down Fang's legs. Jeff felt his mouth pop back into it's original "o" shape, and sighed with relief.
The neighbor's dog sniffed with interest at Fang's new addition. Then, still sporting his member, he jabbed it into the hole.
Jeff felt the most unbearable pain, and realized that Fang's new vagina and uterus were the equivalent to his dick and balls.