Hedgehog in the Fog is a 1975 Soviet/Russian animated film directed by Yuriy Norshteyn, produced by the Soyuzmultfilm studio in Moscow.
This is a story about a little hedgehog (voiced by Mariya Vinogradova), who is on his way to visit his friend the bear cub. As explained in the introductory narration, the two would meet every evening to drink tea from the cub’s samovar, which was heated on a fire of juniper twigs. As they drank their tea, the hedgehog and the bear would chat and count the stars together.
On this day, the hedgehog decides to bring the bear cub some raspberry jam. As Hedgehog heads out, a sinister looking eagle-owl is following him. Hedgehog passes through the woods and encounters a beautiful white horse standing in a fog. He is curious as to whether the horse would drown if it went to sleep in the fog. The hedgehog decides to explore the fog for himself…
The fog effects were created by putting a very thin piece of paper on top of the scene and slowly lifting it up toward the camera frame-by-frame until everything behind it became blurry and white.