I think it was a typo. I think jvanrens meant “the horse that wouldn’t pace a lick without hopples will under saddle.” The horse was a pacer, a Standardbred used for racing. Pacers wear hopples, a loosely-fitting bit of harness on their upper legs that encourages them to stay in a pace and not break into a trot (or a gallop, I would assume). I think of it as a martingale for the legs. So the horse in question would not maintain the proper gait even while wearing hopples until they started riding it, at which point, it paced naturally under saddle.
@jvanrens Did I get that right?