Is it normal for a horse to foam at the mouth? surprisingly, yes!
There is a saliva duct in the throat latch area of a horse (behind the jaw, where the head attaches to the neck) and when a horse is tense or carrying their head incorrectly the duct does not work adequately, creating a dry mouth. A moist mouth is preferred and actually helps the bit work better. When a horse relaxes and rounds and lets the head just relax off the poll, then that saliva producing duct flows freely and creates the saliva which forms into foam as a horse mouths the bit. Foaming at the mouth can be a sign of a relaxed, happy horse and should not be an issue of concern unless a horse does it when not working or exhibits other unusual symptoms.