The phrase “hold your horses” means: Don’t be in too great a hurry; take it easy; watch your step; keep your shirt on; be patient; control your temper.
This homely admonition traces back to the American county fair of old, to the races which, among the menfolk especially, were the main features of the day.
The harness races were especially difficult to get started, for the horses, sensing the tautness of inexperienced eager drivers, were constantly breaking from the line and had to be called back.
Figurative transfer to human restiveness and its restraint was but a step.
As early as 1844 we find in the old New Orleans Picayune, “Oh, hold your hosses, Squire. There’s no use gettin’ riled, no how.”