I'm running across this a bit late, but I thought I would throw in my 2 cents worth. With a horse you need to establish that you are leader of the herd, with a dog you need to be Alpha in the pack. Dogs are still essentially pack animals and most of the time think of us as pack leaders. If you can think with that mindset (or the mindset of any animal in it's natural state) you will have a better chance of getting it to do what you want.

Like P8nt, the dogs that went with us, stayed with us because they wanted to. That is not to say they didn't occasionally follow a good scent or chase a rabbit or squirrel (dogs gotta have fun too!). But the most important thing was they understood when you gave the call or whistle for them to come back, that was the end word, no discussion, no arguement.

I have always taught all my animals to obey commands instantly. In the case of a friend's dog, it saved her life! (Queensland Healer in a rural area, gave the down command while crossing the road and a car drove over her with no injuries.) This requires a lot of training and complete trust by the animal. But after seeing the importance of complete obedience I began to do the same with all my animals -- well okay, nothing a cat does is instant LOL -- my cats DO obey simple commands, they just complain and grumble about it the whole time. >^,,^<

If you don't have obedience off lead and off trail you won't have it when they are riding with you.

The other thing we taught all the dogs (most were med/large australian shepherds or smaller breeds) was to ride the horse with us without fear or fuss. This is important in case the dog injures a paw or something and needs to be carried back home. Again, do the training ahead of time. Finally, always carry a little food and water and first aid stuff... just in case!