Anime: Zou no Inai Doubutsuen 3e6n7

After the war, concerned brother Hide promises his critically ill sister Miyoko that he will arrange for her to see a real elephant. But when he goes to the zoo to find one, an old man tells him of the tragic day when the zoo animals were slaughtered.

Source: The Anime Encyclopedia

Note: the story is based on true historical events that took place in the Ueno Zoo in Tokyo, where dangerous animals were slaughtered during WW2 in order to prevent them from wreaking havoc in case Allied bombings would let the animals escape.