Horse phrases

Would like to know the origin of: "changing horses in the middle of the stream" and "see a man about a horse".

Summarizing from previous discussion, the phrase "see a man about a horse (or dog)" relates to a person being "unwilling to reveal the true nature of his or her business."

FROM THE ARCHIVES:

I've written previously about some of my grandfather's expressions.

It also occurs to me that when I used to visit my grandparents as a child, my grandmother and grandfather had different ways of referring to going to the toilet. He would say "I'm going to see a man about a dog" and she would say "I need to spend a penny". I can guess the origins of her expression but am less clear on the origins of his. Any ideas?

Could both be classified as euphemisms? Are there many similar expressions?

Yes. "Going to go see Miss Murphy."

To explain the phrase for non-native English speakers: When someone says that they are going to see a man about a dog they really mean that they are unwilling to reveal the true nature of their business.

The expression comes from the long forgotten 1866 play Flying Scud by a prolific Irish-born playwright of the period named Dion Boucicault. One of the characters uses the words as an excuse to get away from a tricky situation. This character, an eccentric and superannuated old jockey, says: "Excuse me Mr. Quail, I can't stop; I've got to see a man about a dog". This is the only thing that seems to have survived from the play.