… or a horse.
I grew up using “I’ve got to see a man about a dog” to mean “I’ve got to do something, but exactly what is none of your business.” The first time I heard the version of the phrase with “about a horse” I thought it was a riff on the “standard” phrase with the size of the animal increased for humorous effect – nope, turns out it’s just another version.
Which version is the “real” one as far as you’re concerned?
I knew a Texan who always said “he had to see a man about a dog” whenever he headed to the bathroom.
He was the only person I ever heard say this, and nobody ever knew what he meant.
Good friend of mine uses, “… I have to turn my bicycle around”. Clever and funny in a nonsensical kind of way.
I’ve always heard/used both as a(n?) euphemism for going to the bathroom, going off to do some ummm “illicit” activity, or as you say twicks when it’s none of anyone’s business what I’m going to do.
Then there are my comedic Uncle Marty’s ways of expressing it:
“I’m going to see a dog about a man.”
“I’m going to see a horse about a man.”
“I’m going to see a dog about a horse.”
“I’m going to see a horse about a dog.”
A laugh a minute my Uncle Marty is! 
Hm, yeah, I guess it usually is about heading to the john. I was thinking about it because I said it to someone at my current contract gig when I had to leave early yesterday to go see another client; she was all “I thought you had two cats, why are you getting a dog?” Turns out she knew the “horse” version and had never heard “dog.”
My friends father said he needed to see a man about a horse. We all got excited because we took it literally, being little kids and all.
We thought he was going to buy a pony.
After that he used dog, cut down on all the excited kids.
Never heard dog, horse plenty of times. It usually means going to take a leak, but could be referring to other functions.
Horse is the one I grew up with. I don’t know if I’ve even heard the dog version.
Now, if you’ll excuse me, I have to go say howdy to a cactus.
I once told an overnight guest that he had to leave in the morning because I had to see a man about a horse, and he thought I was blowing him off, but in actual fact, I really was going to look at a horse for sale. Nice little paint gelding… bought him, too.
Horse or dog, I’ve always known it as a euphemism for needing to relieve oneself. I was under the impression that ‘horse’ was a sly reference to the size of the man’s anatomy.
I’ve always heard “I’ve gotta see a man about a horse.”
I’ve always heard “horse” and it has always meant going to the john.
Ditto.
Shit, never thought of it that way. I feel … sheltered or something. Normally I have a dirtier mind than this! slinks off in shame
I had an ex-boyfriend who said he “had to see a man about a mule” when he was going to buy pot.
We used to use “have to give birth to a Bosun’s Mate”, but that wasn’t for taking a leak.
Oh, and “Leaning over the rail to tell Ralph about my Buick” meant the seas were rough and greasy pork chops were on the menu.
Melbourne, Australia.
Always, only, about a dog.
I’ve rarely heard horse, never dog. Never stopped to think about what it might mean, either, though I did realize it wasn’t about an actual horse.
Atlanta, Georgia, USA.
Always, only, about a dog.
I mean come on y’all? A horse?
I’ve always heard ‘about a horse’ no specific meaning other than 'I’m not really going to explain where I’m going.
Since American Psycho came out I use ‘I have to go return some video tapes’ to escape encounters I’m uninterested in staying at.
Gotta go drop the kids off at the pool.
Or, Gotta go pinch a loaf.
Grosses the kids out every time!