A lot of polo ponies are cutting or Argentine gaucho ponies.
These breeds make great low- to medium-goal polo ponies, but most high-goal players are on Thoroughbreds or Thoroughbred crosses. Only animals with a good degree of Thoroughbred blood have the speed and stamina to gallop the long distances required in polo.
High-end polo ponies are, in fact, bred specifically for polo, from successful playing stock. Polo requires not only a level of physical ability and strength, but also a certain ‘heart’ that can’t be judged when analysing potential polo mounts.
You need a pony who is brave, will attack a play, but still be obedient and willing, and breeding from proven stock increases the genetic probability of this ‘heart’ being inherited.
While breeding specifically for polo is an excellent plan when the time and resources are available, it is also a risk. Unless you clone, you cannot affect the genes specifically to breed for the qualities you want.
You can hope that the offspring has the best qualities of both sire and dam, but no equine is perfect, and imperfect qualities can be bred into offspring. Regardless of the genetics, you will have to wait at least five or six years to know whether the progeny will make a top-notch polo mount.