iluvwalkers, I am so glad to hear that your foal is doing well. that was so lucky for him, that you were there...maybe not so lucky, had you just run down to the store, or over to a friend's for coffee...
I see that you have taken offence to those who have tried to warn you of the dangers inherent with dogs and horses. Please, sit back, take a breath...and listen to what has been said.
Nobody is trying to make you out as a bad owner, of either the horses, or the dog...so if that's your worry...then let it go. As for KNOWING your dog...yes, perhaps you do, as much as we humans can...but what you don't know...is what that dog was thinking as it grabbed that foal hard enough to leave marks, on it's neck. Think in terms of escalation...
I also see that you are relatively new to this Forum, and I welcome you to the best place I know of, where you can learn so much about this wonderful breed. Please, take the time to browse this forum for "Dog attacks", and that might make you see just why everyone has "jumped into your business" here...and has so much to say.
Me? I have been lucky with my dogs. I have never had one who was allowed into the paddocks/pastures with the horses, and so far, they have all recognised the boundries. BUT...should I find myself owning one that doesn't...it shall be found a new home, without dhildren, or animals. In my opinion, a dog that has learned to mishandle animals, will eventually do the same with small children, and it just isn't worth the chance to me...to find myself proven right or wrong.