Bella has fear aggression towards certain dogs, generally it's if they charge over to her without the proper doggie etiquette and then she'll react by air-snapping at them. With some large dogs if she spots them her tail goes up immediately and she'll start barking (I usually manage to nip this in the bud by just keeping an eye on her tail position and as soon as I see it go up, I call her to me and distract her).
Her fear aggression stems from having been attacked 4 times by staffie/bull terrier types in her first year. She was for a while aggressive towards any dog falling into that category (she would actually cross the park to go and do her "come on then if you're 'ard enough!!!" routine with them), but thankfully after months and months of work she no longer does this.
Zorro's arrival has helped to calm her down as he is very good with other dogs and will also go and back her up if she's looking worried. We've recently had a set back as a Rottie harrassed them both, the owner refused to call it off, and when we put the dogs back in the car the Rottie the paced round and round, jumping up at the windows, frothing at the mouth and scaring them both silly
. So with big dogs Bella is now quite frightened again but we're working on it.
The only time Zorro ever growls at another dog is if it tries to take his ball.