How did the term 'see a man about a mule' originate?

Brad Pitt in the movie Kalifornia.

This phrase originated from the old saying " see a man about a dog " which at the time simply meant that, dogs were bought and sold quite regularly, however being that it was considered a daily chore it became a euphamism for going to see a woman for sex, or in broader terms any act you would rather not reveal or have publicly known.
So if asked " where are you going " you say " to see a man about a dog" which means the same as the more familiar " see a man about a horse ", not so much to mean going for sex, but to use the restroom, another act you would rather not publicise, but definitly not as severe as what it used to mean.

This phrase is actually found in a book called "Their Eyes Were Watching God" by Zora Neale Hurston, first published in 1937.