Too many pads or pads which are too thick allow the saddle to roll. A stack of pads can also cause the back to retain a lot of heat. Whatever type pad you use, the shape of the pad should match that of the saddle. Especially in hot weather, you don't want to cover any more of the horse's back than necessary. Breast collars, cruppers, and bridles come in nylon, leather, and Bioplastics.
Ordinary nylon gets stiff from absorbing salt and dirt, and is hard to keep clean.
It is available in many widths and colors, and more and more suppliers are making horse tack out of it. Traditional leather can cause chafing and will need regular care to stay soft and pliable. However, leather will break in case you have a fall, and this is a good safety feature. Serious accidents can result from the rider being hung in the tack. A good breast collar can help hold the saddle in place, preventing it's sliding back or roiling sideways.