In response to oc_spirit:
I can't quite remember exactly what happend at the kennel since it was about two years ago. I don't believe that Diver and Shadow were taken down the hallway together; just one at a time. All the dogs were barking very loudly like they always do when new dogs come in. The kennel didn't say anything had happend like any fights breaking out. I don't believe any of the dogs were put together; I think they all had seperate kennels and seperate play times. I didn't think anything of it because immediately after nothing serious had happend. But little by little Shadow had become angry and growling at Diver. And the only thing that ever happend in Shadow's life that would even point to the cause was the kennel incident.
Shadow was actually from a family oriented breeder. The woman said that she wasn't selling the puppies for high prices (I think my mom and I only payed $300-ish) because she wanted them to be family dogs. He was always with his mom and before we even got him he was already potty trained. He was very sweet and cuddly from the very beginning. Diver was the one that was from a puppy mill. I'm not sure if she's a reputable breeder. It sounded as though she had bred before because she sounded as though she knew what she was doing and knew what she was looking for in the owners. No young children to be loud and tail-pull type of situations.
As far as the fights themselves. At first they were just to knock each other down. A lot of noise and spit, some fur. Then it got pretty bad. Diver has really soft skin, velvety fur so he is scarred very easily. He has bite scars all over his front arms, face, and ears. He's beautiful and shiny, but sometimes he looks as though he could be a professional fighting dog, which I feel so sad about. One time when I wasn't around my mother exclaimed that when the two rushed at each other Shadow got a chunk from Diver's head and the piece of skin was missing. Not too big; more of like a 1/4"X1/4" gouge. We took him to the vet to see if they thought it needed stitches, which it didn't. But the last time when it was really bad Shadow was trying to bite Diver's ear and got his tooth stuck behind more on his neck and I jumped toward him and took hold of Shadow's mouth to keep him from ripping and Diver accidentally snapped and got my hand.
I can tell Diver is scared but is always willing to fight for his place I guess. Because he always tends to shake afterwards like his nerves are going crazy and he can't keep himself calm. I feel bad for him, but then I feel bad for Shadow. I can't take sides, and I don't want to give either away.
It seems like both tend to start the fights. They both stare at each other and glare, bark and growl when passing each other's cages. Diver seems like he never takes his eyes off of Shadow because he's afraid of a sneak attack or ambush. Diver gets anxiety and Shadow is calm.
Yesterday I looked into ordering a greyhound-like muzzle.
Just to even try and socialize him without any risk to the other dog. I would start with Diver of course.
An embarrassing story my mom told me one time. She wanted to walk Shadow around the neighborhood and I guess one of the neighbors had a loose dog. He must have been a terrier size but my mom said he just Picked him UP! It didn't sound like anything bad had happend; he dropped the dog right away. But I mean, seriously! It seems kind of funny but then scary at the same time.
Sadly now, most the people in the neighborhood know now to stay away from Shadow but can approach Diver.