My current project has me looking up medieval & early modern horse diets, including horsebreads.
Horsebreads were generally coarse ground grain and some form of legume, baked into a fairly dense loaf. The refrain on most (of the very few... horsebread isn't glamorous research) mentions of horsebread was that it was the same bread eaten by peasants, simply because it was cheap. Although it was undoubtably cheaper to produce and to procure than refined white breads, I have a feeling the legumes (peas, fava beans, etc) played a much larger role than simply making the bread inexpensive. Peasants, i.e. laborers, needed protein just as much as the horses did, and neither had access to a meat diet.
Many of the available recipes favor rye, even when horsebread started to be refined to be fed to racehorses. Interestingly, rye has a much better calcium:phosphorous than wheat or even oats. Very roughly, these are 1:2, 1:8, and 1:4, respectively, with ideal being 1:1 to 2:1. Assorted peas and beans also have better ratios than wheat, though again they fall short of the ideal (for examples, peas are roughly 1:4.5, and fava beans- once known as horsebeans- are roughly 1:4). When wheat was used (and it usually was), it would be sifted and the finer flour would be reserved for more expensive breads for human consumption. The finer flour contains more simple starch, while the coarser (which includes most of the bran) contains more fiber and more protein, all of which is better for the horse (and the peasant, even if it was less appealing).