I'm finding a hard time getting through the description. If it's the entirety of the character as an idea then we are talking about beast-to-human evolution. Beside a few Isekai themed series, it's mostly present in manga and light novels. If we go for young beast-child to mature-human form then it's easier to find something like this in the light novel medium. If it's light novels I'd recommend fiddling around with the novelupdates site search function and tags until OP gets what he searches for. Even so, I'm getting downvoted while trying to understand what exactly OP is searching for .. I'm never learning my lesson to stay as far as possible from anime or manga based subreddits ..