Seeing a man about a horse is at least 30 years old. I can remember my grandfather using it from time to time.
Here are some refs I found:
TO SEE A MAN ABOUT A DOG (OR HORSE) - "Although in the late nineteenth century, to 'see a man about a dog' meant to visit a woman for sexual purposes, it now means to go to the bathroom. It is, of course, a traditional answer to the questions Where are you going or What's your destination? The variations on these expressions are endless and include: Go see a dog about a horse, go and see a dog about a man, go and shoot a dog, go and feed a dog, go and feed the goldfish, go and mail a letter and go to one's private office." From the "The Wordsworth Book of Euphemism" by Judith S. Neaman and Carole G. Silver (Wordsworth Editions, Hertfordshire, 1995).
Or
To see a man, to see a man about a dog, or to see a man about a horse is usually a smiling apology for one's departure or absence, used as a bland euphemism to conceal one's true purpose. The phrase has several meanings but all refer to taking one's leave for some urgent purpose, especially to go to the bathroom or going to buy a drink. During Prohibition, "to see a man about a dog" often meant to go meet one's bootlegger. The phrase "see a dog about a man" is also sometimes used.
http://en.wikipedia.org/wiki/See_a_man_about_a_horse
From the urban dictionaries, it means:
Any general business that needs attending to that you may not care to discuss whith the present party. A discreet way to leave the table at a restaurant to go to the restroom; derived from a 1940's black-and-white film (sorry, cannot recall the name) where an actor literally says it as he leaves a table to see a man at another table about a horse.
http://www.urbandictionary.com/define.p ... ut+a+horse
Hope that helps.
Sometimes the best thing to say is nothing at all.