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I was going to suggest Fantasia, but the Pegasus creatures in the film do not talk, or do I remember anything about a boy talking to a unicorn. I do not think it is the Lion, the Witch, and the Wardrobe, though I think if I remember right it did have a Pegasus. There is a 1973 short called Pegasus, which has a farmer inventing a Pegasus, but it is only eight minutes. Are you sure it was a Pegasus not another winged creature like a dragon?
There is Princess Gwenevere and the Jewel Riders, but that came out in 1995, it features a unicorn befriending a princess, and the unicorn is raised in a family of Pegasi and is seen as an outcast.
Edited by thestormtrooper

When I was a little kid, I watched an animated fantasy film. It's a traditional animated film to be exact. It's released in the late 1970s. I only saw a few clips of it and I can remember a few scenes. One clip shows a boy flying on a pegasus over some giant flowers. Another scene shows the boy talking to the pegasus who gives him words of advice. I don't know the name of the movie but I bet you can know for sure.
Edited by Astropilgrim1621