Livestock blower.
Gets the horse clean CLEAN clean of dust and dirt in the coat, a zillion times faster than the vacuums ever will.
You will have to use a currycomb to deal with crusty things or anything sticky on the haircoat.
But for $400 you can get a new sullivan blower. These things last years, no bags to change. My sensitive horses that do NOT like grooming have no problems once they learn about the blower.
They are hard to find used, like a good Pony-Club saint beginner pony is hard to find, unless you are in the loop and in the right place for it to be passed down. They do show up on Craigslist, but I’ve only seen one in like three years. But they do last forever.
You can’t use it if it’s really, really cold. But if it were that cold, you’d be taking a blanket off a clean horse, right?
And it will get plenty of dust and dirt blowing this way and that, so you wouldn’t do it where the dirt would blow, say, all over the tack in the tack room.
I got a Sullivan’s Air Express II for Christmas two years ago. I LOVE it. And I hate horse vacuums.