What is the origin of the phrase "I need to see a man about a horse"?

Factual Questions
21

[QUOTE=Surok]
That’s the one I know, as well, so perhaps we can assume that’s the British variant. And someone, when it crossed the pond, the dog mutated into a horse.
[/QUOTE]

The floridated water did it.

22

[QUOTE=FRDE]
I’ve never heard it about a horse

  • but ‘I need to see a man about a dog’ is familiar.
    [/QUOTE]

From the first cite in the second post: