Some horses make strange lip movements seemingly just for fun. However, lip movements can also be indicative of a wide variety of emotional states and can even signal a physical problem.

Like lip curl (Flehmen Response), a variety of other lip movements can be considered normal behavior or can be a sign of a physical pain, or anxiety or irritation.

Lip twitching and trembling can be related or unrelated to eating. Occasionally, a horse's lips will move in a strange or inappropriate way when they are offered feed. These horses may also not be able to properly grasp feed with their lips and move it back in the mouth. This is a classic finding in horses with Yellow Star Thistle or Russian Knapweed poisoning.

Foreign objects embedded in the mouth and dental issues can cause strange lip movements out of discomfort.

Lip twitching and flapping are common after a horse has been given an oral medication or de-wormer. The lip curl (Flehmen response) is a natural gesture when a horse is presented with a new smell, and is common in breeding stallions around mares. I wrote another record dedicated to the lip curl.

Some horses flap their lips loudly when they are nervous or anxious, or anticipating an undesirable event.

Recognize that a variety of lip movements can be considered normal, but that they might also signal anxiety or even a physical problem.