Brindle is an extremely rare color pattern in horses, and is not yet understood genetically.
It consists of irregular "drippy" stripes extending vertically over the horse's body, as well as horizontally around the legs. Brindle horses also have a dorsal stripe.
Salsa, a brindle mare owned by Brenda Charpilloz - as an adult (above) and a foal (below left)
Because the striping patterns are similar to dun, it is sometimes confused with dun, especially in minimally brindle animals. For a long time, brindle was considered to be a form of dun.
For a more thorough exploration of brindle, see Sharon Batteatte's Brindle Horse Website
So far, Brindle has evaded breeders' attempts to breed for it.
RARE & MISCELLANEOUS: BRINDLE BROWN CHROME CURLIES WHITE DUN OR BUCKSKIN? CHIMERAS AND SOMATIC MUTATIONS
GENETIC OVERVIEW EQUINE BASE COLORS DOMINANT GENES DILUTION GENES RECESSIVES