I apologize in advance for the length of this post- I want to try to get all the pertinent information in.
I currently have seven dogs (five are mine, two are fosters), and six of them are no problem at all. The seventh (mine) I just cant seem to get a handle on. I feel like I am missing the third number on a combination lock. Im sooo close, but I cant get it open

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I trained horses for many years, so I am familiar with herd (or pack) mentality...how members of the pack subtly test for rank, and try to move up in the pack. I understand that it is important to be the dominant member in the relationship, especially with a dominant animal, and I think I accomplish this most of the time.
I adopted the problem dog (a lab/australian shepherd mix, about 100 lbs.) a year and a half ago, when he was 11 months old, and brought him home to be a companion to my 12 yr old lab mix. He has a VERY strong prey drive (killed a gopher the first week), and I could see even then that he was very strong both mentally and physically, so I have kept a firm hand on him from the beginning.
The first day the old dog tried to take the bone he brought with him (my mistake), and he went after her, so I took him by the nape of the neck and put him on the ground. (I know from reading this site that is not recommended, but he has never shown the slightest human aggression. In fact, when he is at home, I can put him on the ground with a hard stare.)
At home, I have very few problems with him. He and the old dog have had a couple of power struggles (she used to be top dog), but in general he does what I tell him, and he does not have conflicts with my other dogs. I have even seen a small foster dog stick its head in his food bowl, without him doing a thing (I removed the dog, of course.)
So much for Dr. Jekyll.

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Mr. Hyde comes out when I take him anywhere in public. I started out walking him with a choke chain, but I might as well have been using a string. Any time he saw another (dominant) dog on the trail or in a yard by the road, he JUMPED at them, and it was all I could do to hold him back, and not be yanked over.
So I got a Halti for him, and it helped quite a bit to be able to control his head, and keep him from sniffing the ground and getting excited. It was a familiar technology for me - I have halter broken a lot of horses, so know how to use the leverage effectively.
In fact, I thought he had progressed enough for me to be able to take him to a park for an adoption dog reunion. I put both the halti and the choke chain on him, and I thought I could control him, and desensitize him to strange dogs, by having him lie beside me while the other dogs moved around (this approach worked well with my small dogs.) I was wrong.

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Instead, in the course of the reunion he lunged at other dogs four or five times, and twice succeeded in actually pulling me to the ground. He did not get away from me, or succeed in attacking the other dogs, but he seemed to get progressively worse, rather than better. as the day went on. He did not attack all dogs, just certain ones, and I dont think he actually wanted to hurt them, just put them in (what he considers) their place.
I could break his concentration at first, but by the afternoon, he was redirecting his hostility toward my other dog when he was not able to reach his target. Obviously, this is NOT acceptable.
So...this is where we are at, now. I just received my dominant dog collar in the mail, and gave it a test run on him in the yard. I considered ordering the prong collar, but he is already excitable, and after reading more, I decided against it.
I would love to hear any thoughts from people who are more familiar with big, dominant dogs than I am. I have had a lot of dogs in my life, but this one has been the biggest challenge I have ever had.