What do you call a dog with no legs?
It doesn't matter; he's not coming.
Listen up, buttercup. The joke is funny because it plays on the expectation that you'd give a dog with no legs a name, but then hits you with the punchline that it doesn't matter what you call him because he can't come anyway. The author thinks you don't know a dog needs legs to come, so he must think you are really dumb, like dumber than a dog without legs.