Why are white hairs scars?

A white mark on a horse's back or leg in an odd place, or of an odd shape, often means an injury - sometimes horse people might call them scars, but we more normally call them "marks."

Pressure injuries to a horse's hide, and more rarely cuts, can damage the hair follicles and cause them not to produce pigment. The hair will then grow back white, usually permanently (in a few cases it can be temporary and "grow out" the next time the horse sheds). In modern times there's quite the market for "touch up" sprays which temporarily dye the hair back to the horse's coat color for shows.