Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. My own dog is/was very much like that too, and she also is a dog that we rescued from the streets of Mexico. If everything else is fine. . .she's not particularly anxious for example, then the best training then the best training techniques to use are desensitization and diversion.
Having her around as many dogs as possible is desensitization and this is excellent. Diversion will be training her to sit or lay down upon command and stay with treats for successful behavior. In other words, when you are around other dogs and she starts to get aggressive, you give her one of those commands and give her a treat when she obeys.
Does Ruby know those commands?
We have taught her sit but it doesn't always work. When there are other dogs around she won't respond to anything at all and she won't take treats outside at all.
Thanks for your response! It does take more training with some than others, and it definitely takes consistent training. . .on a daily basis! You might be helped if you and Ruby attend a dog obedience class. The instructor will give you commands for you and Ruby to work on, and will be able to evaluate how well things are going. Ruby could be "a hard nut to crack", as was my dog Scout, but once she understands that your commands must be obeyed in all situations, then she'll be much more compatible with other dogs.
The other advantage to training, is that she'll learn that you are in command and that she can trust you. Often it is helpful to command her to sit and stay after you put the food bowl down, and when she obeys you release her and with lots of praise. You become the "food god". . .her provider.
I did think about dog training classes but putting Ruby in a room with other dogs is not an option. We do make her sit before she has her food (the same as we do for our other dog) and she does do this. In the house she will sit and stay and responds really well and she gets lots of praise and her favourite treat. It's just when we are outside all the problems start. She won't sit, she won't respond to her name. It's like as soon as she is outside the door she is taken over by a demonic beast. In 4 months of trying she hasn't improved at all when she's out. She doesn't care about anything except finding a dog to harass.
I hear you!!! Demonic beast is very descriptive. The only things I know to do are to make sure she gets lots of daily exercise (chasing balls or sticks) until she is somewhat tired, and then use desensitization and diversion. There are board certified veterinary behaviorists at larger specialty clinics or veterinary colleges that you could contact. Also, another expert on here may have more appropriate suggestions for you. I'll "opt out" and this puts your question back on our queue for other experts to pick up. Don't respond to me, as this just puts you back on my list alone. I wish you the best of luck. . .these dogs can be tough, but highly unlikely to be impossible.