I'd say it depends on where you live, and whether you're talking about critter-type enemies or weather, disease, etc. I live in Wyoming, and the worst enemies to horses here are mountain lions and feral dogs. The mountain lions are a more recent problem, as several years ago non-native wolves were introduced into Yellowstone Park and the entire state does not have enough habitat for these animals. The mountain lions and other large predators were driven out of their secluded homes by these invaders and are now found preying on ranch animals and living dangerously close to towns. Feral dogs are a huge issue because they're not afraid of humans and have discovered that livestock is easy prey, especially horses that are in smaller pastures or kept temporarily in corrals.