Eating red clover can cause a horse to drool excessively. It is not harmful to the horse; it is just a little alarming to see a horse drool. If your horse is grazing red clover, then simply removing it from the red clover field will cause the drooling to cease. However, teeth or mouth problems can also cause a horse to drool. Occasionally, a horse may get something lodged in its throat which may cause excessive drooling so it is always wise to have a veterinarian examine your horse to eliminate all possible causes.