Horses sweat all over their bodies, unlike many mammals (e.g. dogs). Sweating is an the skin’s response to over-heating which occurs after exercise or if the horse is too hot, e.g. hot sun or over-rugging (a common problem). Horses are phenomenal athletes and running fast can cause phenomenal sweating, but also mental exertion can contribute, e.g. when doing dressage. Excessive exertion can cause hyperthermia, with body temperatures over 40 degrees, especially on hot days, and rapid cooling is required to avoid physical problems (e.g cardiac arrythmia).
Abnormal sweating can be all over and can occur if the horse is distressed physiologically, such as colic or overt pain, such as a fracture. It can also occur in patches,
Non-weight bearing on a limb, limping at walk, limping at trot, stiffness, limping only occasionally, resisting certain manouvres, not jumping certain jumps (spreads/drops), tripping, disunited at canter,