My neighbor has a dog that is the terror of the neighborhood. It is very aggressive to other dogs and squirrels. I had Pixie, the cat my ex-room mate abandoned, on a leash and was taking her for a walk in the yard. The dog ran up to us and Pixie puffed up to about twice her normal size and attacked the dog. Scared the crap out of the dog and it ran home and scratched at the door to be let in. Now when it sees Pixie come out it runs and hides. It's the funniest thing.

BTW - Pixie is the sweetest cat ever and loves every human she's ever met.