Geez, I wonder what strange planetary alignment keeps bringing these soap related questions?
Well anyway, as far as horse shampoo’s go their main benefit is that their made for horses, in other words real simple stuff. No added perfumes, no balsam, protein, vitamin E, collagen, coloring etc. and without this added stuff horse shampoo rinse out really well and dissolve dirt and oils great…
A little TOO great. See, unlike a horse’s coarse hair, without some kind of lipid (natural fatty oils) your hair would get to be all dry and brittle and nasty.
Most of the folks who prefer horse shampoo (my grandpa uses one called “Tail and Mane”) because it dissolves better in mineral rich well water (I also suspect they don’t shampoo every single day or that they condition separately afterwards).
Finally, dog shampoo is fine on horses, especially clean rinsing ones like “Top Paw”.