Also, a horse of another color. Another matter entirely, something else. For example, I thought that was her boyfriend but it turned out to be her brother? that's a horse of a different color. This term probably derives from a phrase coined by Shakespeare, who wrote "a horse of that color" (Twelfth Night, 2:3), meaning "the same matter" rather than a different one. By the mid-1800s the term was used to point out difference rather than likeness.
American Heritage Idioms