ALL of our horses MUST accept dogs in and around their area. The only exception is in the horses stall. I say this because I live in an area where stray and free ranging dogs are the norm, and it is just part of life. Also most if not all of our horses are used to some extent or another to work bird dogs. And I have 2 rather large livestock guard dogs that are everywhere on our farm.
I have found that introducing the dog and horse through the fence, or on lead leaves too much of the situation in your hands, as the moderator, or alpha..... My suggestion, which has worked for ALL 17 of our horses plus a few burried out back, and at least a hundred dogs that have come and gone from our place, is to let the dog and horse figure things out on their own. If you have a big enough pasture id say let them intermingle on their own. I promise within a short period of time the dog and horse will work out a set of boundries they can both live with. The only thing is that you need to be sure that the dog is smart enough, and fast enough to get out of the way if your horse decides to charge. They will never feel eachother out with you dictating every encounter.