High Withers can be an obstacle to getting a good saddle fit. A horse with a "standard" conformation will have defined withers, but some horses' withers are especially long and angle backward to create a steep ridge descending to the back. High withers are more prominent in thoroughbreds, saddlebreads, and warmbloods, but can show up in other breeds as well.
Abnormally high withers can actually improve your horse's performance because of its increased ability to lengthen its stride. This means that once you get the right saddle fit, your horse may outperform the rest. Getting the right fit is easier for a high-withered horse because the symptom is more common than other hard-to-fit conformations.
Important Points to note while saddle fitting a high-withered horse:
The basics of saddle fitting still apply. Just because your horse has high withers does not mean your saddle can be resting on them. The saddle should still clear your horse's withers almost an inch (2-3 fingers). If the saddle is sitting too high on your horse (more than an inch off the withers), the saddle is too narrow. If the saddle is riding down less than an inch or touching the withers, the saddle tree is too wide. But proper saddle fit does not only depend the withers---it also takes into consideration the width of the back and shoulders. If your high-withered horse has narrow shoulders and a wide back (or in other words, your horse is pear-shaped), you may wish to try a gaited saddle. Gaited saddle trees are narrow in the front and wide in the back.
There are many pad options for high-withered horses. A cutback saddle pad has an opening for your horse's withers, allowing them to come through and not putting any extra padding around them. Two popular options are the Big Horn Tuffy High Withers Pad or the Tucker Toklat Wool Blend Pad with wither cut out.