The continued dissection of the Left 4 Dead 2 PC demo turns up a reference to other possible features in the game: three soundfiles entitled "zombiedog_attack."
Steam user McEnvy emailed me that screengrab. (The full size screen is below, showing the directories and other information). Looks legit enough to me. He sounded familiar with the demo and its contents so I have no reason to believe that is fake.
However, as we heard from Chet Faliszek yesterday, other files found in the demo were test files or placeholders, and not content in the game or coming up. These files also could be leftover assets for a feature that Valve ultimately abandoned or removed from the game.