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I do have prints showing French horses with bound tails, so perhaps that was just when on the battlefield, or maybe just a mistake by the artist. I guess unfettered but well groomed tails look better on parade.
I dont really know when these images were made, but tied tails seem to be fairly common. For example see http://www.directart.co.uk/mall/more.php?ProdID=6437.
In the common Osprey books look at
"Napoleons Carabiniers" page 14
"Napoleons Dragoons and Lancers" page 11
"Napoleons Cuirassiers and Carabiniers" page 6
I just wondered if there was any regulation about this.