Back in the 80's, one of my aunt's used to record a bunch of cartoons off of TV onto VCR tapes for my cousins watch over and over. When they would get bored of them, they would give me the tapes.
One of the cartoons I ended up with was a feature length film about a little unicorn. I don't remember much about it, except that at the end the antagonist was like somehow (I assume magically) turning all of the other unicorns into blocks in some big wall or structure or something. And somehow our hero unicorn was trying to stop him... Honestly, I'm not even sure that it had a happy ending, or at least maybe it was a bittersweet one. It was in English in the US.
This would've been '85 to '90 range when I watched it. It would have been recorded sometime within a year or two before. I know this is obscure, but I've thought about that cartoon a few times over the years and could never figure out what it was.