Something a lot like that happened to me. I stopped and complained to the owner. "Whats a matter? You afraid of a little doggie?" "No, but if I crash avoiding it I get hurt and you get a hairy fat lawsuit. So, a person is worth more then a dog, you have been warned, and I am not going to swerve the next time it chases me. This MTB can crush a dog that small. You may not care but think how your kids will feel." Those two little yap dogs were never seen out of their yards again.

In your specific case notify the park rangers and Animal Control each time it happens.