A Montana Department of Transportation public safety video advises viewers, “If you plan to drink, plan to have a friend get you home.” This being Montana, the “friend” in the video is a horse, raising the question whether it is legal to drink and ride. Apparently it is. Montana law only prohibits operating a vehicle while under the influence, and a horse is not a vehicle.
UPDATE: Professor Bainbridge notes that, in some states, a horse can be a “vehicle” and riding drunk can result in a DUI.