Grunt is another form of vocalization. It's a way for horses to communicate, just like humans use words and gestures. Horses grunt as a way to indicate their physical state (pain, distress, anger) or to let other horses know that there's danger and they should be alert of their surroundings. It can also be used for training and communication, or even to attract a mate.
Nevertheless, there are many different reasons why horses grunt. Horses make grunt noises when they are in pain or are frightened. If the horse feels pain while being ridden, he will grunt to alert the rider. Horses can also grunt if they have injured themselves or been hurt by another animal. Horses may grunt to warn their owners about dangerous situations such as lightning storms, fireworks, or loud noises. A horse can also grunt if he feels a bit uncomfortable during exercise. This could be due to too much work, too hot weather, or too tight reins. Pay attention to this sound and act accordingly to prevent possible further issues.