This is Reckless Dan, an unusual grey brindle Quarter Horse. Most brindle horses have dark stripes on a lighter body color, but a rare few have white stripes on a dark body color. While it is possible that white brindles are chimeric (and certainly some may be), the current thought is that the color is probably the result of a one-time mutation, probably of rabicano.
An odd one indeed - I wish there was another picture showing the full body of the horse. Could be a Tobino (because of the facial markings) with an interupted White Face and the inner ears are also white.