I use this rather puzzling phrase to announce that I’m going to the john, or at any random time, really, if I’m leaving a group of people. But I don’t know what the hell it means. Anyone?
I always took it to be related to “Gotta pee like a racehorce”–derived from the use of Lasex in racehorces, stops their noses from bleeding during the exertion of the race, makes first time users run somewhat faster. Lasex is a diuretic, so it makes Mr. Horse pee. A lot.
So, along the lines of the above link, “see a man about a horse” is a “discreet” reference to pee like a racehorse.