He read more of the story. As he was reading it he realized that he made a mistake. He would turn into a black stallion, and if it followed the story, he would too be picked up in half an hour by a truck and horse trailer. Then he would be led to a barn.
Then he grabbed the wand and started make a wish but dropped it because his hands were forming into black hooves. His feet started to get uncomfortable and so did his pants. So right before his hands changed completely, he took off his shoes and his pants and boxers. He felt uncomfortable about being naked, however fur was spreading all over his body. Mark wasn't able to get the shirt off due to his hands changing into hooves.
Then he looked down as his feet started changing into hooves too. Next to change were his legs which the knees reversed, and he felt onto his front hooves. Next his nose and mouth started stretching outward. Then the final completions of the change finished. Afterwords he grabbed the wand in his mouth and tried to make a wish to change back.