Passive mobs spawn VERY slowly, but they do spawn. However, they will only spawn if the passive mob cap is not maxed out, and passive mobs do not despawn. That's why the animals don't seem to change in an area under normal conditions. If you're not killing them, nothing new is spawning. Thus, if an area doesn't have any horses it never will have any horses unless you kill off the existing passive mobs to make room for new spawns. However, even then horses spawn at a much lower rate than anything else, so it will take a long time.
I will do so if there's there's some kind of bug with the server that's preventing them from spawning. However, if it's simply because they are very rare and hard to get, then I think it should be appropriately difficult to get them.
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