If your horse is a young foal, it may only be shedding its "baby" hair before growing in its normal coat. If it is an older horse, you may need to know if it has been kept under lights to simulate long day length. Horses kept under "long days" for an extended period of time often become "refractory" to the photostimulation and will begin to shed at unusual times. If this is not the case, you may wish to have your veterinarian inspect the horse for any skin disorders that may induce hair shedding.