High tail: to get away fast. When a horse runs and are excited, they usually lift their tail high or straight up!
Nightmare: An angry mare can be a scary thing, especially at night!
Hem and haw: When driving a horse (with a wagon or cart) hem is to turn left and haw is to the right. If you hem and haw, you are not progressing forward. Sometimes in thought also.
Got your goat! To keep a race horse calm, a goat is often a companion to the horse while in it’s stall. If someone gets your goat, then you are upset.
Horsepower: Most people understand horsepower, but the average horse can produce about 15hp. However, the horse is the fastest thing on the planet from 0 to 40 feet. They can reach 40-45 mph in less than two strides. (Not donkeys).