A beautiful 1975 animation by Yuri Norstein. I recently saw it for the first time and it just blew me away.
Hedgehog and his friend bear cub have a nightly ritual where they drink tea, count the stars and have conversations. On his way to bear cub's house, hedgehog sees a white horse, and wonders whether she will choke if she lies down in the fog. He decides to investigate and delves into the fog, becoming hopelessly lost.