- Media: Movie
- Date of release/watch: I watched it a couple of years ago, but the movie itself came out a long time ago (definitely pre-2000)
- Plot summary: A mother (in the form of a horse) tells her child a story and thereby narrates most of the movie by describing events and telling stories. It starts by her telling a story about a king dying and the line of succession (two or three brothers fight to get the throne).
- Setting: The movie doesn't really have a traditional setting, and this is due to a peculiar way of storytelling (see Additional info below)
- Additional info: It's a very psychedelic animation, with a black background and bright primary colours. The characters are merely coloured shapes, and they will go from humanoid-shaped to horse-shaped on a whim, with a lot of explosions of colours and flashes. The sound design is also quite psychedelic. Another detail is that once or twice near the beginning, the mother telling the story quite sternly hushes her child. I know it's available on Youtube, too.
- Country: Hungary (produced and language)
You are looking for: Son of the White Mare (1981)
Original title: Fehérlófia
Son of the White Mare (Hungarian: Fehérlófia) is a 1981 Hungarian animated adventure film directed by Marcell Jankovics.
—Son of the White Mare, Wikipedia
Fehérlófia (Son of the White Mare) is born as the third son of a horse, and he gains his power by suckling the horse's milk. He listens to old tales, mostly about the Forefather and the end of his reign, caused by evil dragons.
—Synopsis (Son of the White Mare), Wikipedia
Fehérlófia is a breathtaking piece of animation. Jankovics uses a color scheme of audaciously bold and vibrant colors mixed with glowing pastels. The result is a dreamy kaleidoscope of entrancing hues. The style is fluid, as shapes unfurl seamlessly into other shapes. The frames are symmetrical, psychedelic visions of ancient memory with all of stylistic innovations of the last century.
—Fehérlófia : The Most Beautiful Psychedelic Trip You’ve Ever Seen, HuffPost
I know that you said it is a pre 2000 movie, but I feel like I should just present this finding and make sure with you that it is not it.
How about Symphony N. 42 (2014)?
The film presents 47 observations in the irrational connections between human and nature. The film applies an unconventional narrative. It presents a subjective world through 47 scenes. Small events, interlaced by associations, express the irrational coherence of our surroundings. The surreal situations are based on the interactions of humans and nature. Differences between human and animal diminish, everyone is doing their jobs and leaving traces. Every small movement affects another, building an unpredictable, irrational system.
It looks abstract as you described with vibrant colors and shapes becoming humans and animals. Plus, it is Hungarian. Unfortunately I could not find a horse yet, but the word "horse" is between the keywords.