Megan was standing behind Will, and had her arms wrapped around his waist as she reached down between his legs. One of the first stages of Will's change was when Megan reached his cock and found first that it had grown too large for her to reach the tip, and too big around for her hand.
Then something made Megan jump. She was standing behind Will, but she could feel something fleshy start to poke into her and grow larger. As she jumped back, she could see a tail forming from the base of Will's spine, She watched, fascinated, as it grew longer and hairier, until it reached its full size and started to switch back and forth behind him.
Will reached up and scratched his head, which was itching. He found that his ears were growing pointed and taller and hairier. He turned around to face a mirror, and saw that he had gained a fine hairy pair of donkey ears.
Megan started to laugh, and Will's ears swiveled at their bases, following the sound. "You've certainly made an ass of yourself this time, Will!" she giggled. She looked between his legs, and found that his balls had grown to match the new, more massive size of his cock. She also noted that his cock had darkened in color and had changed subtly in shape, not entirely equine, but certainly no longer 100% human.
Will was not so amused, and wondered at what point - or even if - the changes would end.