Responses
This is not a one-shot but the description is really similar,
http://www.mangago.me/read-manga/seibetsu_mona_lisa_no_kimi_e/
Yeah not this one, but it sounds interesting
I think it's a oneshot. There's this were-beast kid lost in a forest? And picked up by an older guy. This kid is genderless, and supposed to choose gender the moment they are a certain age.
(Spoiler)
The kid always wanted to be female so they could be together with the guy. But in the end decided to be male to protect the guy, since being a male will make them way stronger :((