Well?
A bit of poking about on the interweb seems to suggest the first written use of the phrase so far located is an 1866 play called “The Flying Scud”. I haven’t found any source that suggests that play actually coined the phrase, it may have been in popular verbal use before that. Or not.
Before someone else says it, use Google next time my man.
We’ve kicked this around before. And I’ve been guilty of using the dread phrase “Google is your friend” my own self.
But after a number of spirited debates, most dopers seem to agree that telling people to use Google instead of asking their questions here is really sort of counter-productive to the purpose of the SDMB.
Even if the answer is easily found, it will lack the give-and-take repartee, the commentary, the arguments, the various anecdotes that often accompany queries here.
Just my two cents, which carry absolutely no weight of authority.
If you would take the simple trouble to google “Qadgop the Mercotan” I think you’d find that you carry considerable authority around here. Google is your friend, my friend–please don’t fill up this thread with silly, erroneous nonsense when a little searching saves us all so much time and effort.
Well, despite opinions to the contrary, I’m wit’ youse!
Thanks, guys. I’ve been wondering about this phrase for a while, and then it turned up in A Song of Ice and Fire, of all places. I just had to find out, and google didn’t even occur to me.
Forgive me, but I gotta ask: If you feel this way, why are you wasting your time and effort on this thread? 
<<whoosh!>>
PRR, thanks. Compliment received on this end.
I’m so confused!
I say the OP’s horse line all the time,never even thought about where it came from.Good Question, Septima.
Now,excuse me, I gotta go take a shit.
The “silly, erroneous nonsense” prr was referring to was Qadgop’s plainly wrong statement that he “carries absolutely no weight of authority” around here.
Clearer? 
Oh.
::slinks off into the sunset::
Maybe I’m the only one to take DIGGLEBLOP’s post as a whoosh.
He has been beat to death over the last few months with the ‘Google is your friend’ line.
Pay back time, DIG?
One of his better posts in GQ. He was right to post it.
I’d always thought it was mule, not horse, and I first heard it in the movie Kalifornia.
Really.
I’ve never heard it about a horse
- but ‘I need to see a man about a dog’ is familiar.
Just for the hell of it:
“I need to see a man about a horse.”
–Cotton Hill, from the FOX animated series, “King of the Hill.”
How common is this phrase, as that’s the only place I’ve ever heard it?
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.