Yukiguni no Ouji-sama

Koichi and his sister Yuki befriend Hanaguro, a swan who has flown to Japan from Siberia. Hanaguro is attacked by a dog, and the cowardly Koichi flees, but that night he has a dream in which he is whisked away to the Kingdom of Snow Prince, where he finds the bravery to help the other swans.

(Source: The Anime Encyclopedia)