Sooty, also called Smutty, or Shaded
Many, if not most horses have some degree of this, which is most likely a form of
natural camouflage, consisting of a layer of black
or other dark hairs over the center line of the back, the hips, the shoulders
and sometimes other parts of body, as well as the mane & tail.
These dark to black-looking hairs grow "on top of"
another color, usually gathered together into particular areas forming
"smudges", usually near the top line; no doubt a form of camouflage,
apparently designed to disguise the top, normally-highlighted areas
of the horse.
If another animal were looking for a nice big, round, meaty
meal, it would look for a big, rounded shape, defined by the light
reflecting off the top surfaces. When these surfaces have
darker hair on them than the rest of the shape, they don't
register as top surfaces, and the shape of the horse is effectively
Look at the old Quarter Horse in this picture. He has
dark patches on each hip bone, to minimize their appearance.
His big, round barrel (rib cage) is also counter-shaded... dark
where there would naturally be highlights. The same thing is
true of his large, noble head: the top, which would normally catch
the sun, is darker than his muzzle, which would normally be in its