Roarer - a horse with laryngeal hemiplegia.
Quote:
“The condition is caused by a nerve dysfunction affecting the left side of throat. The nerve dysfunction results in a muscle that fatigues easily and can't hold the throat open when the horse exerts itself. The left side of the throat collapses, restricting the airway and causing the telltale "roar."”
I think jmclaugh has already pointed out that unbeaten 1886 Triple Crown winner Ormonde, considered one of the best racehorses to grace the Turf, was a roarer.
(There have been at least eight racehorses called Borealis. If one of them was a roarer people probably called it a roarer Borealis.)