To Get One’s Goat means to aggrevate.
Origin: Hyperactive racehorses were often given goats as stablemates because their presence tended to have a calming effect on the horses. After the horse became attached to the goat, it got very upset when its companion disappeared, making it run poorly on the track. In the 19th century, when a devious gambler wanted a horse to lose, he would get the horse’s goat and take it away the night before the race, thus agitating the horse.
With the Belmont Stakes coming up this weekend, you want to make sure that there’s no gambler trying to take your horse’s goat.