I have always taken the term “horse hockey” to be synonymous with “equine feces.” I’ve also heard the term “bull hockey” with a similar meaning.
What I have not been able to find, though, is how did “hockey” come to mean “feces” in selected usage.
Any thoughts???
Okay, this may be a stretch, but bear with me.
I’ve always heard it as horse or bull puckey. I’d assumed it was because of the shape of the leavings; note, though, that I’m not a specialist on equine or bovine feces, so it is indeed possible that horse and bullshit are not, in fact, shaped like a puck.
But assuming they are, it would be simple to transpose puckey to hockey, giving rise to the turn of phrase you mention.
And the problem with small furry animals
in corners is that, just occasionally,
one of them’s a mongoose.
Terry Pratchett, Witches Abroad
“Vandelay!! Say Vandelay!!”
About thirty years ago an old Virginia Gentleman (FFV, I’m sure) said HORSE HOCKEY in from of us and from the way he jumped back and apologized we knew it was POWERFUL language and used it whenever we could.
I remember hockey specifically because of the ice game and then later wondering if it was what polo players said when the game was about to begin like “Play ball” or “Ready set go” for then it would be: “Hi ho and Horse Hockey!”
Horse-hockey is from the early 20th century. When children could find time, and makeshift hockey sticks, they would play hockey on the streets in the winter.
Of course, back in those days, delivery wagons were pulled by horses. Vehicle pollution did not go up in the air like it does now.
Not having a lot of money for sporting equipment, children improvised.
A frozen piece of horse turd, could be cut and shaped to the desired shape, size, and weight, for a decent hockey projectile.
Wow, DEAD zombie resurrected here. But it’s worth adding that I grew up with a couple of horses and we called them horse ockies.
I think you must be thinking of cow flops, aka cow pies. 'Orse ockies are too small and shaped to cut down to much of anything.
I always thought it was because playing ice hockey on horses is impossible. If your description is true, my faith in humanity is falling. 
How far back does it really go? I always assumed it was just something made up by the M.A.S.H. scriptwriters so Col. Potter could sound like he was swearing without actually doing so. I have never heard (or read) it anywhere else. Do real people actually say it? Did they before M.A.S.H.?
Bull / cow feces are generally very liquid and make the typical splat / pie formation.
Horse scat is generally more solid and formed. Not to the extent of, say, rabbit or deer turds. But approximate size and shape of a golf ball. Thus “road apples.”
(/grew up on the country.)
I don’t know where it comes from. I suspect that its resemblance to the sport “hockey” is coincidence, and not the result of derivation.
Vance Randolph records “hockey” being used for “feces” (including human feces) in the Ozarks in the 1920s in his books of folktales. I’ve occasionally heard it used in other places since (Col. Sherman T. Potter once used in on MASH*).
Random data point: When I was a kid, we said “ockey”, without the “h”, for poop.
Here’s one take on the etymology:
I originally drafted a post to say just this (although naturally much more wittily and humorously). Then I noticed Flypsyde’s post wast over a decade old and I didn’t remember his username from any recent thread.
Besides, golf balls ? You must have only been around constipated nags. Tennis balls is more like it. And that’s if the horse is considerate - most barely make an effort and shit outa fibery mess with some balls in it (NSFW if your coworkers assume you’re into both beastiality and scat play).
Our zombie friend doesn’t know what’s what. The specific term “horse hockey” to mean “excrement” doesn’t go back to the early 1900s. It’s from the Korean War period. Which makes “M.A.S.H.” not unbelievable,
Hockey, to mean excrement DOES go back to the1880s or so.
