Posted by Ian Kusz on July 31, 2007
I am seeing the answer to this in several places, and have an alternative possible explanation. I cannot verify, for sure, how far back the tradition dates of color coding riders and horses, but the tradition of men following hounds to hunt dates back to the Ancient Middle East. According to answers.com, the Romans followed the practice (presumably for entertainment, as they also imported a type of non-native hare and additional species of deer to use as quarry species to England) in England, though it seems to predate their presence. By 1340, the traditional hound-hunted animals include the hare, the hart, the wolf, the wild boar.
Since it is not really possible to keep up with dogs chasing deer on foot, ditto wolves and rabbits, it is presumable that one did it on horses. In fact, by 1650, according to American sources, people were importing special horses and hounds to America to hunt the (non-indigenous) red fox. George Washington and Thomas Jefferson are said to have been involved in this type of hunting.
Since Shakespeare was alive from 1564-1616, and this practice predates him, it may be possible that the quote, "A horse of a different color" comes from this activity.
It is traditional (and I cannot verify how far back the tradition goes) to color-code the hunters and their animals; the best riders wore a different color than the average rider; younger riders wore a different color, and horses were color-coded to denote traits of concern (green for a first-time rider, red for a difficult animal, prone to kick).
So, stating that "that is a horse of a different color," that is, distinguished by an alternative color (on the rider or horse). Since it can be presumed that a fractious animal would respond differently than a well-trained or less-capable mount, and the behavior and performance difference between a complete novice and an elite equestrian can be quite marked, the saying seems quite appropriate to this (possible) origin.
I haven't been able to verify if this is the origin, but it does seem a possibility, considering the dates of origin of this pastime. Can anyone verify how far this tradition goes back? I'd love to know.