Sometimes you might see a picture of a horse with an apparent checkerboard pattern (or a more complicated one) on its butt.
These are called quarter marks. They're created by placing a stencil on the horse's hind quarters and brushing over it against the grain, causing the hair to stand on end in a pattern. (A really skilled groom can sometimes apply them without using a stencil).
Quarter marks are traditionally seen on show hunters and riding horses/ponies in the UK. They're intended to help highlight the quality of the horse's coat and if done right can just make the horse look better.
Canadian Mounties often put maple leaf shaped marks on their horses for parades and other formal occasions. Sometimes you'll see national emblems on horses at the Olympics, college mascots on horses ridden by college equestrian teams, etc. Kids (or parents) often put cute designs such as stars or flowers on their ponies.
Quarter marks don't hurt (or even annoy) the horse, and they look pretty - at least until the next time it rolls.
This picture shows traditional checkerboard marks on a French cavalry horse taking part in a parade. (Source: Marie-Lan Nguyen).