Where in the world does the phrase "I have to see a man about a horse" come from? I feel I ought to know, since I use it all the time.

Going to "see a man about a horse" - or "a dog" evolved from the 19th Century as an excuse to get out of an unpleasant situation.
Before coming to mean "using the bathroom" it was also used at different times to refer to seeing a woman for sexual purposes, or even to mean going for a drink (during Prohibition).

These sites offer a little insight:

http://phrases.shu.ac.uk/bulletin_board/10/messages/441.html

http://www.briggs13.fsnet.co.uk/book/d.htm
(About halfway down the page)

http://www.word-detective.com/072999.html
(6th reference down)

quote:

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Originally posted by Texan-In-Exile: ...also used at different times to refer to seeing a woman for sexual purposes...

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Thank you!
I'll tell you what, though, if my husband ever starts using that phrase to me, this is one horse that ain't puttin' out.

The common use now is as much
a form of male braggadocio, an
implication of phallic proportions
equal to that of a horse.

I think it's just a conventional excuse, 'I have to go somewhere now'.

It's like saying to a pesky little kid, 'I think I hear your mother calling you."