Horses were once had a vital role in human life, used as a primary means of transportation and the main source of power in agriculture. Although they have lost those roles, their legacy remains in the English language.
We still refer to the purpose of many parts of the horse and its equipment. If a person is champing at the bit, s/he is eager to get started or involved, the bit being the part in the horse's mouth. For that matter, unbridled enthusiasm is not tempered by caution since a bridle limits a horse's movement. In that case, someone has to rein in, i.e. pull back or restrict the freedom of movement. Sometimes, the opposite is necessary, meaning you to have to spur someone on, referring to the sharp point at the end of a cowboy boot used to get the horse to run faster. To help focus, horses and athletes need blinders to avoid being distracted. Of course, it is hard to be calm if you are saddled with worries or debts. Many people almost succeed but it has no meaning since almost doesn't count unless in horseshoes and hand grenades, the former being a game in which you try to throw a horse as close as possible to a stick, a bit like les boules in France. If you have to hoof it, you got to walk, which has nothing to do with hoof in mouth disease, not thinking before speaking and making embarrassing comments.
Even in general, the horse maintains its presence in language. Engines are measured by horsepower, technically 550 foot-pounds per second. If someone tells you to hold your horses, you need to stop immediately, with an image of an out-of-control four horse carriage popping up. Before an eagerly awaited date, many guys are hot to trot, all sexually excited. Of course, an employee can be a workhorse, strong and dependable, or even a thoroughbred, carrying all the right genes to be a top manger. Still, they may need a hand up, some help, derived from assistance to getting up on a horse. In the end, all employees are put out to pasture and retired, which is not always a bad thing. After all, you shouldn't look a gift horse in the mouth, meaning you don't ask questions about gifts. That assumes that you have any horse sense, which is basic wisdom so common in horses and rare in humans.
To end on a proper note, to quote Cuthbert Soup, “You can lead a horse to water, but you can't make him participate in synchronized diving.”