Aunt Ruth is right. The problem with most drawings I they don't use the whole value scale. Dark darks will help tremendously. Also remember if you need something to come forward put dark shadows around it evan if there are not any showing in the photo you are working from.
I use a mechanical pencil. Because I am a lefty, I have a hard time using the graphic pencils due to how easily they smudge.
This is my first time truly shading horses, so any help I can get is good!