I have met some dogs while in costume, and have gotten different responses. Both good and bad.
The bad one was last year when I saw a Labrador, and tried to say hello to it. It totally freaked out, barking, shoving its teeth, and backing away. This was in a big crowd of people.
Another dog I tried to talk to, started looking around, not knowing where the sound came from, apparently not recognize me as a human. Another acted quite normal though and let me pet him, so I guess you never know. Just be careful, and see how it reacts before you get to close.