The term 'Horse Controller' is a job title for someone who is responsible for the care and training of horses. The term can also be shortened to 'REIN, ' which is a four-letter word that is often used in crossword puzzles. Horse Controllers are responsible for the overall health and well-being of horses, including grooming, feeding, exercising, and providing medical care. They may also be responsible for training horses for riding, racing, or other activities.