a horse of another color

An entirely different matter. In Twelfth Night, Shakespeare had a character say, “My purpose is, indeed, a horse of that colour,” meaning “the very thing.” It's not difficult to see how that phrase would be changed to “a horse of another color” to mean the opposite. Other formerly popular equine expressions are “don't change horses in midstream” (stick to your original plan); “get off your high horse” (don't be so proud or smug); “don't beat a dead horse” (don't waste your time); “one-horse town” (a tiny rural community); and “horse show” (derisive college slang term for a debutante cotillion).