Polo wraps are cloth "bandages" that are sometimes wrapped around a horse's legs for protection and support while being ridden. Some show jumpers use them. The name comes from the fact that they are used in polo to protect the horse's lower legs from being hit by the mallet.
Polo wraps cover from the knee or hock to the fetlock joint - i.e., the horse's "shins." They are sometimes applied over fleece padding. Usually, they're brightly colored (and often coordinated with saddle blankets). Kids' ponies might even wear ones with superhero logos or flowers on. White polo wraps are sometimes used in dressage demonstrations or clinics to make the movement of the horse's legs more obvious.
This horse is wearing red polo wraps to match its saddle pad. (The stuff over his eyes is a fly fringe). Image source: BS Thurner Hof via Wikimedia Commons.