Can you drive a vehicle into the pasture? Maybe leave it parked there and let the horse get used to it being there? Rde around it. Then start it up and let the horse follow it, ride with it, and then towards it. Rev the engine, honk the horn, back up. I would do this over the course of a few days. I have found this to work with other scary things...umbrella, clippers.
I'm not a fan of road riding but I know many people do this. You need a horse to be accepting of this or it's a danger to horse, rider, and driver.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger