The picture above shows two horses. The horse on the left is not laying its ears flat down, but turning them back, which isn't usually a threat.
Horses sometimes lay their ears back, or just turn their ears back. Just turning their ears back means that your horse is listening to you or something else that's behind it or in the distance. However, if your horse lays its ears back flat down to its neck, that's a warning sign, the horse may kick or bite, so watch out! When a horse lays its ears back, it usually means that the horse feels angry or threatened by something such as you or another horse. Just turning the ears back means that your horse might be listening to something, or it might be feeling sleepy or bored. It's usually not a threat, but when the horse lays the ears back to his neck, stay away, or watch out!