She is an 8 year old working cocker spaniel. It has happened a couple of times, when I tied her up outside a shop for a few minutes, and someone approached her. If it's women or children, it's fine. All tail wags and friendly, loves being stroked. She is a very pretty dog, and people often approach her. But recently when approached by a man; she had a snap at them.
I am now obviously worried about taking her anywhere where she has to be tied up, which is a total nuisance, because it means her getting far fewer trips out than she otherwise would. She's always been a little bit nervous, and she occasionally snarls at other dogs she meets if she doesn't like the look of them, especially male dogs who show a great deal of interest. But she's never really caused a problem before, and she has never shown this aggression to humans until very recently. We took great care to socialise her as a puppy, and we did puppy training. What can I do now? Is it a case of getting a muzzle? Can I retrain her to stop doing it?