Dude I had that thought last night. I was laughing because I have a headcanon that because of Lucifer and the left over grace in him, Nick can see a lot of supernatural things that most people can't see. And Hellhounds is one of them.
And the hellhound, being raised by a human on earth, is more affectionate and like a normal dog, than a hellhound raised in hell. So there was this image of someone knocking on the door and the hellhound getting excited and running toward the door and Nick has to run after it and hold it by the scruff of it's neck and answer the door with one hand. And the person at the door can't see the hellhound, so Nick's just holding something invisible and he's like, "Uh, it's my dog. Heh." and they look at him like he's fucking nuts because there is nothing there. So they have him sign for something he does and and then he shuts the door and looks at the hellhound like, "I gotta train you better, you huge dope."