The hollow cupping sound of a horse's lips flapping together can be a source of great amusement for people. But a horse may develop this habit for reasons that are not benign. So take a minute to try to determine what might be behind the behavior.
If your horse flaps his lips while standing quietly in his stall and seems otherwise relaxed, you can probably just chalk the habit up to quirkiness and leave it at that. However, if the behavior is new and is accompanied by pacing, pawing or other signs of agitation, it may signal discomfort, colic or stress. Keep an eye on the horse, take his temperature and call the veterinarian if you suspect he may be ill.
If your horse is flapping his lips due to anxiety or boredom, the remedy may be as simple as providing more turnout time or forage, but you'll also want to rule out any physical cause. Ask your veterinarian to perform a full oral exam to ensure your horse's teeth, gums and tongue are healthy and not causing discomfort.
If a horse flaps his lips while being ridden, it is almost always an indicator of stress. (Remember that active lip-flapping is different than the simple drooping lower lip of a horse who is extremely relaxed.) Step back and examine your riding and training regimen, perhaps with the help of an unbiased trainer. Consider whether your horse may be overfaced by a specific exercise or anxious about a particular event or activity.
Keep in mind that the lip-flapping may start immediately after a stressful moment or begin in anticipation of it. When you think you've made an association, adjust your training to reduce his stress levels and see if the behavior subsides.