I'm currently collecting data on animal/horse/plant spawns outside of the map since we don't really have any data on it. We all know the odd general location to grab a horse, but not much else. Hopefully this will be helpful to us in some way, and well, it is the "mapping thread" after all.
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I've found that the horse spots are pretty random, but they are still within a radius, so they are more like "spawn zones" rather than the traditional "spawn point". More data is still needed though.
Here's a little video of some of the most reliable horse spots I've found.