Horse
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.