Breed, appearance or size of dog is irrelevant. As long as the dog is trained to perform a task to assist a disabled person, it gets access. It can be any task. I remember a service dog that was trained to carry a bottle of water and make sure that its handler drank water.
ADA applies only to two types of animal - dogs, and in certain circumstances miniature horses. It doesn't apply to any other animals. Even if someone has a service cat or a service monkey, a business isn't required to admit that animal by law.