Tack stores and catalogs are full of hundreds of different pieces of horse training equipment, but what do you really need to successfully train your horse? Every gadget claims to be the answer to all of your problems, but the truth is that keeping things simple is the best approach.
Horse training falls broadly into two categories: ground work and under-saddle training. Trainers use ground work to teach a horse basic skills like leading, lunging in a circle, and picking up its feet. Ground work is also valuable for teaching a horse some of the things it needs to know to be a safe mount, including carrying a saddle and responding to rein cues and pressure against its sides, where the rider's legs will hang. Under saddle training further refines these cues while improving the horse's physical condition and athleticism.
For ground work, all you really need is a halter and rope. Most halters are made of wide leather or nylon straps, but halters tied from a single length of 1/4-inch nylon rope are more effective for teaching your horse not to lean against pressure, since it is more uncomfortable to lean against thin rope than a wide, flat strap. Ropes should be thick and heavy enough to feel comfortable in your hand, and have a strong, but easy-to-open snap for attachment to the halter. Inexpensive lead ropes for horses are often only six or eight feet long, which is a bit too short for many training exercises. Keeping a 12-foot rope and a 22-foot rope on hand will let you achieve most of your training goals on the ground, though some trainers also use a lighter 45-foot rope for advanced lunging and ground driving techniques.
The last -- and completely optional -- piece of horse training equipment for ground work is a stick or whip that acts as an extension of your arm. Many fiberglass training sticks have a leather or rubber loop on one end for attaching a short, thin length of rope or a small flag made of cloth or a plastic bag. The rope extends your reach with the stick, allowing you to reach your horse from up to ten feet away. The flag is useful for sacking out a horse that is uncomfortable with flapping or noisy objects.
Under-saddle training requires some additional equipment, including, as you might guess, a saddle. Depending on what type of riding you wish to do, you might choose an English saddle, a western saddle, an Australian saddle, or even a sidesaddle. Regardless, it is very important that the saddle fit both you and the horse. A badly fitting saddle is as uncomfortable for your horse as carrying a heavy, ill-fitting backpack would be for you. Likewise, trying to ride in a saddle that doesn't fit you will make it almost impossible to maintain the proper position. Because a saddle is a major purchase, choose a good-quality one and measure it carefully. Better yet, try to get the saddle on a trial basis, so you can use it for a few days before committing to the purchase.
To protect your saddle investment and make things even more comfortable for your horse, you will need an appropriate saddle blanket. If you've done a good job finding the right saddle, you shouldn't need any fancy, therapeutic pads. Basic saddle pads serve two purposes: reducing friction and keeping the bottom of the saddle clean. Choose a simple design in high quality materials and keep it scrupulously clean.
The final piece of horse training equipment that you need for riding is a bridle. Bridles consist of the reins, headpiece, and bit. A simple snaffle bit is a good choice for most training; curb bits and other specialty bits may be appropriate for advanced training in some disciplines, but if you find yourself contemplating a harsher bit to overcome control problems, it's probably time to back up and figure out where your training process started to go wrong. If you want an even gentler option for basic training, consider a sidepull or bosal -- two types of hackamore, or bitless bridle. Be aware, though, that not all hackamores are gentle. Avoid any hackamore that involves leverage or shanks.
With a basic selection of ropes, a good halter, a training stick or stiff whip, an appropriate bridle, and a well-fitted saddle, you will be prepared to tackle almost any training situation. Whenever you are tempted to purchase a training gadget to solve a problem with your horse, ask yourself if the problem could instead be solved with more knowledge or help from an experienced trainer. You may find that going back to gentler equipment and simpler training methods is ultimately more effective than the latest martingale or leverage bit.
Lyons, John, et al. Lyons on Horses. Skyhorse Publishing, 2009.
Parelli, Pat. Natural Horse-Man-Ship: Six Keys to a Natural Horse-Human Relationship. Western Horseman, 2003.