Meet my trusty friends, the noise markings.
These markings mimic photo noise - the pixel speckle. On a low opacity, in either white or black, they will create the appearance of texture and detail by making each delightful pixel on your horses coat slightly different from its neighbours.
See here for a side-by-side comparison!
Your mileage may vary, of course.
I have found it to be useful for adding texture to roans and greys, and I'm sure it has other uses.