Different types of animals show pain in different ways. Horses, as prey animals, have evolved to keep most signs of pain hidden, as such if you notice your horse in pain, it generally requires attention.
A horses normal temperature is 99F – 101.4F (37C – 38.5C), if a resting horse's temperature is more than two degrees above or below, then the animal is likely in pain.
A horses normal respiratory rate is roughly 12 to 20 breaths per minute, if a horse is breathing more rapidly than usual it could indicate a problem.
-Kicking their Belly-
Horses show discomfort in many ways, the most common of which is kicking at their belly, often a sign of colic (serious pain in their stomach).
As with kicking at their belly, horses who have pain in their stomach and guts will often roll about, even thrashing their legs. Certainly horses do roll for fun, but generally not for prolonged times.
Some horses, when in pain will pace often this is out of stress or confusion in reaction to their discomfort and suffering.
When in pain many animals do not feel like eating, and horses are no exception. A horse who is off its feed for two feedings should be checked for problems. These might be related to the teeth, stomach, or can be due to an infection of some sort, check the horse out entirely.
If a horse is experiencing pain in its back, legs, or feet, it may shorten the length of its stride as a way to avoid some of their pain.
Limping is an obvious sign of pain in a foot, limb, hip, or shoulder. Typically if the pain is in the front foot the horses head will rise when the sore foot hits the ground, the reverse is true for hind feet. Limping is generally more pronounced when the horse is trotted in a small circle.
-Resting a Leg-
If a horse is experiencing a lot of pain they will hold a leg in the air or rest it.
-Ears to the Side-
Normally horses keep their ears forward, if angry they are pinned back, but if a horse is in pain the ears might fall slightly at the side, showing the horse is concerned about something.
As with the ears a horse in pain might not be bothered to hold its head up, it will have an appearance of being depressed.
-Change in Behavior-
A horse in pain may not want to be caught, may be difficult to bridle or saddle, and otherwise will not behave as its normal self. To an extreme a horse with back pain may also try to buck.
Become familiar with how your horse is normally as such you will be aware of changes and can spot a problem before it gets too bad. Veterinarian attention should be given to horses in distress, such as when having colic, or showing other signs of pain.