In eighteenth-century England a common way of transporting goods along rivers and waterways was by using flat-bottomed boats called barges.  The barges could be powered by currents or by windblown sails if conditions were right, but when the destination of the goods was against the flow of the water, or the skies were still, it was much more efficient to use horses.  Hitched up to the barges on long tow-ropes, the horses pulled slowly and steadily from the nearby shore.  Massive and powerfully-built, barge horses were bred for their great strength rather than speed.

As the Lady of Shallot watched the daily activity along the river through the mirror in her tower, one of the sights she might  see was these vessels sliding slowly in the water near the "willow-veil'd" shoreline, with their faithful working horses trailing close by on the land.