Sorry for the vague/possibly inaccurate title, it's the closest thing I could come up with for an explanation to my problem.
I have a three year old greyhound/galgo, who we'll call Toffy. Today is his "birthday", actually.
I adopted him as a small, malnourished rescue-puppy, and he was the runt of his litter. Someone had thrown him & his sisters into a ditch. I got him neutered around 6 months, and he was a very well-tempered dog. He grew up with my family's dogs (not other sighthounds), and he got along with all three of them. Shortly before he turned one, I moved out and he was very lonely. He caused a lot of trouble because he missed his friends, so I adopted another dog.
Toffy was somewhat timid, so I specifically asked for a friendly, gentle companion dog. I was given Duchess, a mutt who was living with multiple other dogs at a foster home. Getting her was a massive mistake. I did everything: I had their first visit outside the house, all of their interactions were supervised, they slept & ate separately, etc. Still, Duchess was very insistent on getting in his space, bothering/bullying him, and dominating him.
They had a few petty squabbles over the course of the month, and I started having doubts. The foster pressured me to keep her, saying it would be cowardly to "give up", and a few days later Duchess viciously attacked him while I was supervising their yard-time. Toffy hid behind me at one point, and Duchess mauled me to get to him. She bit me in two places on my hand/wrist, but I managed to herd Toffy inside and lock her out.
I called the foster and begged them to take her back, but they refused. She spent two nights at my house while I tried to figure out what to do, and at one point, she managed to shove past me in the doorway and attack Toffy AGAIN. They were inside, so I was able to split them up faster this time. Distraught, I told the foster that if she didn't take the dog back, I was going to be forced to put her down, since the shelter didn't accept dogs with a history of attacking people. She arrived the next morning to get Duchess, and called me some mean names before leaving with the dog.
Now, that's a long story, and I'm sorry I couldn't shorten it.
Here's my problem: It's been two years since that horrible experience, and Toffy went from a happy-go-lucky, submissive youngling to a defensive, high-strung adult who only gets along with puppies and people. When I visited my family, this switch in behavior led him to fight with the dogs he grew up with.
After seeing that he liked puppies, I introduced a baby galgo who we'll call Bambi, and he likes her. She respects his limits, and she's quite playful and sweet. They have a wonderful time running around together and laying in the grass. Toffy has some resource-guarding issues when indoors, but Bambi is sensitive & avoids conflict with him. She's the perfect buddy, and he's grown fond of her. They even sleep in my bedroom together, and she likes putting her paws over his back when they're standing around.
However, he's difficult in all other regards. I can hardly take him out without him becoming stressed and defensive. He doesn't mind people at all -- it's just other dogs! I muzzle him before taking him anywhere, but I don't like seeing him upset. He's better behaved when I'm not around, but I think that me being present exacerbates his protectiveness.
My neighbor has two giant Spanish mastiffs, and he doesn't mind them -- he actually tucks his tail and boops noses with them through the fence. (I think he's intimidated by their massive size.) But he recently started dogsitting a Cane Corso, and Toffy goes INSANE when she comes around. He becomes very aggressive, and sprints the length of the fence while barking furiously. Usually, he's a quiet dog, but something about his occasional neighbor lights him up!
My question is this: can I work on this? Are there training techniques to work on his defensiveness? It's okay if he's going to be like this forever, it just makes me a bit sad to think he could have doggy-PTSD for the rest of his life
TL;DR: A traumatic incident with a foster dog gave my formerly-friendly greyhound/galgo defense issues. Can I fix this, or should I accept his new temperament?