Clearly defined lumps and bumps (not more generalized swellings) on a horse's back are common, and can result from a variety of causes including saddle induced traumatic injury or other disease processes. The inflammatory condition Nodular Necrobiosis is one of the most common diagnoses for a small firm bump here. A hard skin growth in a gray horse is melanoma until proven otherwise. Hematomas can develop in this area too. Rarely, sarcoids and other skin tumors may also develop on the back. A more rare cause of bumps right down the center of the back is called Dermoid Cysts.

One unique feature of growths on the center of the back is that they can be irritated by the saddle and result in loss of use. Any mass that is likely to be irritated by tack is problematic, and so it makes sense to eliminate it as soon as possible. A definitive diagnosis and aggressive treatment is usually the best course of action.

If there is only a single or a few masses, then surgical removal and submission of the tissue to a laboratory for diagnosis might be the most practical approach. If there are many bumps, a more body-wide approach to treatment might be needed.