It's a good bet that you bought those brushes twenty years ago when you first started riding and haven't replaced them since. Besides being unsightly, old dirty brushes with worn bristles won't get your horse clean. Gummy, sludgy hoof oil, well past its prime, has probably lost all moisturizing ability. But find some time to sort, throw out all those old brushes, cruddy hoof picks, and empty bottles, and start over with new stuff, after all, brushes, hoof picks and the like are cheap. Before heading off to your local tack store, let's review the basics of the grooming box.
Natural fibers tend to be a bit more expensive than synthetic, but both works very well.
Choose a brush that fits comfortably in your hand. Brushes are made for a child's hand and a small hand so pick one you like. Also, know your horse. Some horses are very thin skinned and can't bear stiff bristles. Some horses, on the other hand, are tough guys and crave a scratchy brush.