I believe that most horses have the capability to be dappled. Certain colors (like gray, and sometimes bay, brown and buckskin) are more frequently dappled and I believe it has to do with just their color. Being fat and healthy helps though. I think dapples are more easily recognized on horses who are slick and shiny although sooty horses somtimes show them regardless of weight and sometimes even in winter. I don't know, it is complicated.