Horses are beautiful animals, horses are animals fast, impetuous. But in order to run so tirelessly, the horse needs to breathe. Air enters the powerful lungs through the nostrils. They are large and very mobile in horses. When a horse is worried, when he is tired, his nostrils flare. This is done in order to provide more airflow.
It is this design of the breathing apparatus that gives the horse a long tireless run, it helps to supply oxygen throughout the entire distance to a large body. But the payback is a long, soft palate that overlaps the root of the tongue. As a result, the horse does not breathe through its mouth. The only source of oxygen supply is the nostrils. And if you block them, then the animal will simply suffocate.