When someone is in an unnecessary rush, it's not uncommon for a more centered person in the group to remind the anxious person to “hold their horses.” While the phrase is fairly common, its roots are entirely outdated. 

The term first appeared in Ancient Greek texts, as Homer used it in The Illiad to describe Antilochus's aggressive chariot-driving tactics. As he maneuvered his horses so quickly around a bend that they appeared to run sideways, Atreus warned: 

Antilochus, thou art driving recklessly; nay, rein in thy horses! Here is the way straitened, but presently it will be wider for passing; lest haply thou work harm to us both by fouling my car.

After being cemented in historical texts, the phrase remained a popular way to describe chariot and carriage horses that needed to be stalled, usually to pick up a passenger or let a person out. 

Commanders also used the phrase in battle when men still rode on horseback to confront the enemy. “Hold your horses!” was a way to let troops know that it was time to halt an attack. Though few people today travel in horse-drawn carriages, the phrase is still used to as a warning to slow down or calm down.