Water and gentle soap are the best things to clean horse brush with, however, you'll also likely need other brushes (to comb out excess hair) and a bucket to soak brushes and break up oil and scum deep in the brush. Horse shampoo, human shampoo, Dawn dish soap, and even vinegar can effectively clean horse brushes. For disinfecting horse brushes, use bleach, rubbing alcohol, Listerine, or hydrogen peroxide.