When a horse is referred to as being a “Roarer” it relates to a condition termed Laryngeal Hemiplegia which is caused by a malfunctioning nerve that causes one side of the horse’s voice box to not open during breathing.   This causes abnormal airflow in the throat and the horse makes a roaring sound during faster exercise – hence the name Roarer.   It also reduces the amount of air the horse breathes in and therefore potentially affects its performance during racing.   The condition can be treated with a surgical procedure that permanently holds the malfunctioning side of the voice box out of the way.