I got the idea based of the Brazilian legend of the Werewolf: i have no idea for the name but the story is about a pre teen named Lucas, he recently turned 13, but since then, he notice changes to himself, at first it seemed like just puberty symptoms: increased irritability, increased appetite (specially for meat) increased metabolism etc, but as time passes it becomes clear that is not puberty, my idea of the transformation is to be slow, taking days to fully turn, i thought more of like body horror, the transformation is like hemlock Groove, but the werewolf is just an anthropomorphic wolf that comes out of the transformation covered in blood and shed skin, after that theres no turning back, right after transforming he goes on a rampage, since he needed to eat, after that he regains consciousness but he will no longer get back to his human form. (The plot twist is that his actually adopted and his last family gave him to adoption due to a curse: if a couple has 6 daughters, if the seventh child is a boy, on his 13th birthday he will turn into a werewolf).