To make angry. Many racehorses develop a strong attachment to their stable mascots—dogs, cats, chickens, and, especially, goats. The mascots provide a calming effect— they're the horse's security blankets. One will live in or close to “its” horse's stall and will accompany the horse to racetracks across the country. Horses become very upset when their mascots aren't around, so crafty stablehands would steal away a rival horse's pal. Thus deprived, the horse would become angry when someone got its goat.