I have a 5yo terrier cross and a 7 month old jack russell. Both are lovely dogs, very affectionate and well trained within the house.
However, they are terrible on walks. They will drag me/us all the way to the park then walk nicely when there. If they see another dog, the older one goes crazy with barking, shaking, howling, pulling, snarling. It seems he is very frightened of other dogs but has always lived with other dogs. When he has this reaction the 7 month old joins in barking, but I don't think she's bothered.
The 5yo has a harness and a halti. These work until he see's another dog which he goes crazy at. Puppy has a harness.
We've tried distracting him from other dogs and trying to reward for a calm reaction with toys or treats, but he just doesn't want to know.