Brindle

Then there's the mysterious brindle.  Brindle is the fine, somewhat blurred striping found in some Boxers, Great Danes, Afghan Hounds, the various Bulldogs, etc.

Brindle is believed to come in at least three forms: the chimera, brindling as an extreme form of dun, and other brindling.

Chimera

A chimera is an animal that has two complete sets of chromosomes within its cells, as though it were a set of fraternal twins in one animal.

It's easy to imagine that a horse with this condition could have two different body colors vying for expression.  When neither one succeeds in completely dominating the other, one is visible in this form of striping or ticking against the background of the other.

Dun brindling

When a dun horse has an extreme amount of striping, sometimes it's carried on to the larger areas of the body in the form of brindle-like striping.

Brindle, other

There are brindled horses that do not test positive for being chimeras, and are not duns, either.

Brindling seems to be a very effective form of camouflage, breaking up the smooth, flat surfaces of the animal, and even enabling it to blend in with tall grass, weeds, or trees.

Other reference: until this section is better filled out, I'd like to recommend this web site which has held a fascination for me for many, many years: http://www.brindlehorses.com/