Yaoi one shot where the world was infected with parasites that made people's appearance that of animals/creatures except they change back once per year on their birthday? (one of the main characters looks typically human, the other one has Eagle like characteristics)
It shiuld be this chapter
http://www.mangago.me/read-manga/kemono_jingai_bl/mf/v01/c004/1/
I know the main story is about 2 dads with a kid each who get together. the side story is about the sons. the blonde kid is fat and goes away with his dad to lose weight so he can be with the other kid. he comes back cute & skinny, they get together but then he goes back to being fat.
Ff
I'm so sorry I spent the past probably 30 something minutes searching for the manga I tried really hard... Ff...