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They do spawn. They spawn just as regular animals. Just remember how little regular animals spawn, and the multiply that by the rarity of horses. That's why you haven't found any.

You're going to be severely dissapointed.

have anyone found any?

probably just liars

I've been searching in some grassy planes but on F3 it's labeled as an ocean biome. Would this eliminate the chance of one spawning there?

I found some. At first i killed every animal in my plains biome, then waited. Given the fact that pigs cows etc were taking forever to spawn i decided to travel. I found a black and a black with white spots they had a baby when i found them but it glitched to death on the way home. But yeah it's biome based and what f3 says is what it is even if it looks like something else. and they will spawn in old plains but it will take a long long time. Kinda wish that overspawning bug came back.

Well we finally found some horses! took forever though

You're going to be severly disappointed.