Do you have friends with trail dogs? I'm not claiming to be an expert, but from what I've seen, new dogs pick up appropriate behavior from the pack. We seldom if ever ride with fewer than 3, and sometimes as many as 10 or so (for 2-4 humans). The oldest ones have been going for a decade or so, and 3 new ones joined us this year.
We ride with our dogs all the time, and we have 4 dogs plus we always take Dad's dog Pepper.
As jamesqf said, the dogs seems to learn from each other. Pepper spent years riding with Dad, and she showed our dogs how to run ahead of the horses and stay out of the way. My little Luna still will walk behind my horses, but that is not a problem.
For their part, our horses seem to appreciate our dogs breaking the trail for them. They tolerate the dogs quite well.
We have a little JRT named Whitey, who wouldn't ride with us and didn't like coming down to the barn for the 2 years we have had her. But this last week, she has come with us twice!
I don't have any pictures of horses and dogs together, but here are pictures of one of our many rides with dogs.
Our dogs Whitey, Raylen and Luna, in the truck, ready to go riding! dogs.jpg
Dad's dog Pepper, resting after the long ride to camp: Pepper on Trail.jpg
My dog Luna tired too... Luna on Trail.jpg
Me and the horses Tiff and horses 2012.jpg
Where we go... lake 2012.jpg
Find a friend with a dog that knows the drill and watch it happen naturally!