Shika to Kanta(1992)

Shika to Kanta

One day, Kanta, a lonely boy, rescues an orphaned baby sika deer and names it Poyu. There is something special about Poyu, his golden tail. When hunters go after Poyu, Kanta is determined to protect his golden-tailed friend and the other deer.

(Source: myanimelist)

Adapted from a children's novel with the same name, written by Daisaku Ikeda and illustrated by Sho Murakami. It tells the sad history of overhunting of sika deer by humans, as well as the friendship between sika deer and children.