I am blessed to have a totally dog proof horse. He does not care what they do as long as he can either hear them or see them. They can be wrestling underneath him and he may not like it, but he wouldn't kick or freak out at all.
I have two dogs, Maddie(2) and Luka(1), who love to go with me to the farm. I live in an apartment and I take them with me so that they can run. Everyone brings their dogs and it's a big dog party.
My issue with them is since the horse is so well behaved with dogs they think it's okay to run up behind a horse and sniff him, or just hang out behind him, or just do really anything that a dog is not supposed to do near a horse. Maddie actually has some kind of collie or Aussie in her because she likes to try to herd them. She once got kicked by a not so dog friendly horse while doing that. If I am riding in the ring I have to put them in a stall because she will try to herd me in the stall.
Now on the other hand, they are both GREAT trail dogs. I love love love taking them on trails. Idk how I got so lucky in the staying with me department, because they will both run and play but never leave my sight. Luka - who is a blue tick mix - occasionally will get on the trail of a deer and may take off, but after about 10 or 15 minutes he'll realize we aren't there and sniff his way back.
My concern is that they will try to come up behind another horse while someone is riding them and cause the horse to freak out and hurt a rider. This never had been an issue before yesterday. Often I will take them to the farm and let them just run while I do ground work or groom or feed. Yesterday, however I was there and at first I was by myself, but a friend who lives there saw I was out there and brought her young QH out to ride with me and the dogs I guess thought she was me and started following her horse on it's heels. They listened when I called them back, but it could have lead to a bad situation.
I don't want to leave them at home all the time because they both are so young and friendly and just need to run more than they can in an apartment.