This training protocol is specific to a particular situation but I know I see similar troubles in other homes. Recommendations may need to be adjusted slightly here and there to fit your dog-dog conflict.
This case is one of a multi-dog household where the smallest young dog is picking on the geriatric tiny-dog in the home. It is likely that the conflict was sparked by (1) this puppy’s genetic personality tendencies of being originally bred for herding cattle, (2) she is also the smallest of a group of puppies all being raised together in the home. It was not unseen that she gets pushed over, stepped on, or play was too rough for her with the other puppies. This young dog found that a dog in the home smaller and more fragile than her that she could get a rise out of. This type of one sided bullying fun is often the beginning of damaged relationships between dogs.
If they had the ability to choose their families they would not be living together. Street dogs don’t have conflicts as frequently because they just avoid or move to other areas away from dogs they don’t get along with. Choosing their friends and dogs they like to band with. This is a luxury our house dogs or owned dogs don’t get.
It is not uncommon for me to see young dogs develop into bullies. Most of the time, anecdotally, very young puppies that are put into overwhelming social situations where they get bullied turn into the bully. Sometimes the dog that initially bullied them didn’t mean to be a bully like in the case with this puppy. The other puppies she lives with are litter mates and they play rough with each other. So when one of these puppies play with this little dog they wrestle in the same way. Unaware that fair play means they need to adjust their play style to their playmate.
This young smaller puppy now sees the tiny old dog as an opportunity to practice what she learned and have some fun of her own. The problem is this old dog does not see it as fun and even as pain inducing and very uncomfortable to have this young dog rambunctiously snapping, barking, lunging, pushing, and jumping at him.
Quickly this difference in these two dogs escalates. One getting defensive and more aggressive in their behavior till fights start to break out. It’s all fun and games… Until it turns into a fight.
Something I think is important to mention is that play is only a degree away from a fight to start with. Both are high arousal activities that require the same motor skills. When I have someone who is completely new to dog ownership and has never seen dogs play they will freak out thinking that their dog is fighting. Though typically we instinctively don’t mistake a real fight as play, subtle conflicts between dogs can be easily miss identified.