Horse collar

Put a horse in an ox yoke and any heavy load cuts off its wind. The padded collar spread the load across the shoulders and chest, away from the windpipe, and together with the nailed horseshoe it was in use in Europe by the 9th century. That made the faster horse a real alternative to the ox at the plow and the cart, but only where farmers could grow oats to feed it. So the collar's payoff arrived slowly, through the three-field rotation that supplied those oats, and it belongs on this list as half of a partnership rather than a lone miracle.