Susie wished he could tell his parents the real reason he didn't want to be a cheerleader was that, despite all outward appearance, he wasn't a girl. But he knew how his parents felt about transgender people and had grimly resolved to keep the secret of his gender identity until he could be independent, which looked like it might be a few more years. A few more years of Mom telling him how pretty he could be with a little effort. Until then he spent hours on the family computer looking at transition timelines and fantasizing, carefully erasing his browsing history each time.
Then, while silently and morosely sitting through another family dinner, he received the offer. "How'd you like to skip the whole process?" said a voice in his head. "You wouldn't be precisely human at the end" a picture of a hairy wolfman flashed in his mind "But most unquestionably a boy. I will warn you, though, this is it. Once you've changed, you can never change back."
"Deal" Susie responded mentally. Anything to skip the next few years of "girlness."
He felt his ribcage and chest muscles expand while his large breasts shrank, leaving nothing but nipples perched on his massive new pecs. His clothes ripped. Claws emerged from his fingers and toes and hair exploded all over his body. He felt his jaw and tongue lengthen as his teeth sharpened. The genetic makeup of every cell in his body was altered, as were his internal organs, now those of a male carnivore. Finally, his vagina closed up for good, and his genitals became those of a male.
Susie, now a muscular, hairy, male anthro-wolf, looked at his stunned family and laughed. "Don't worry" he said "I won't bite."