Some horses, especially older horses that are very relaxed, let their lower lip droop markedly. This is usually a normal finding. When these horses become more stimulated, the appearance changes. The key to this is that the appearance is symmetrical on left and right sides.

A normal appearing contour of the face requires functioning facial muscles. Normal tone of these muscles requires an intact and functioning brain stem, facial nerve, and facial muscles. If any of these are disrupted, the horse's facial expression changes on the damaged side. Chewing function is unrelated and is controlled by completely different nerves.

When viewing a horse from the front, you may notice that on one side, the lower lip or muzzle droops or sags more on one side than the other. In some cases, it may appear to be pulled to the left or right. Usually, this appearance is caused by paralysis of the facial nerve on a side. In most cases, there is still feeling. The skin sensation is usually not affected as it is supplied by a different nerve. Facial nerve dysfunction usually results from traumatic facial nerve injury, but can result from a variety of other neurologic diseases.