Vie Long is not the only company making quality horse brushes. While they are more known for their boar and badger brushes, Zenith has been making horse hair brushes for a number of years. Horse brushes are commonly described as a cross between boar and badger. I think of them as initially softer than pure or black badger, but with time will break in nicely. They initially hold more water than a boar, so don't need as long of a soak.
No harm is caused to the horse, the hairs are cut while grooming. Because they are cut, they take longer to break in than a boar brush. Let soak and shampoo several times before use.
One criticism for the Zenith Horse brushes is that they tend to be floppy. These are specially ordered with a lower loft to reduce floppiness. One can always use the traditional Italian Barber pinch, by holding the knot rather than the handle for more control.