Horses must have plenty of good evolutionary traits, because even without human breeding they're very successful creatures. I guess they don't reproduce quickly, so that isn't it. I guess they can't shrug off injuries. They must be really good at not getting caught by predators, so they live enough to have at least a couple of foals.

If a pair of animals (humans included) can raise >2.1 offspring that have >2.1 offspring themselves, that's potentially a functioning species. Now, humans tend not to like this minimum requirement because we frown on dying as grandparents at age 30, but it works fine from a species point of view. That's why the answer to "But how does [animal] deal with [common issue] in the wild" is usually "it dies"