When is a dog not a dog?

Answer 1

Any time it is something other than a dog. See the related questions for additional information on what else a dog might be:

"What is a dog when it's not a dog?"

"Is a dog a dog?" and

"Is my dog a dog?"

Answer 2

When he is a service dog.

Service dogs are not pets; under the law, they are considered to be necessary medical equipment. They can be in public with their handler because their handler has a right to equal public access under the law, and the service animal performs tasks required by the handler to achieve that access.