So I thought up a few L4D3 special infected zombies, and here is the one I personally like the most that I'd like to share with you.
The green flu has worked it's way upon dogs, thus creating the hound zombie. For looks, it has a torn up face exposing flesh on one side of the face as well as what seems to be a large sleep deprived eye (Possibly with a red or yellow tone to it) . On the other side there is a large qauntity of growth all over. It has rotting gums with jagged teeth. The rest of it's body is more exposed flesh, growth and a big unpleasent terrifying sight. To make it really rustle your jimmies, it behaves in a neutral way, as it will follow the survivors around inspecting them, often staring directly at the survivors with it's bloodshot eyes. It will eventually leave the survivors if not injured by them. However, if it is shot at, it uses similar tactics as the boomer. It will release a blood curtling noise that attacts a pack of hounds around you, then will begin to swarm in on you. This requires you to be more careful with a hound following you. They have a basic attack of biting or scratching you. For Versus, it wouldn't be able to attack survivors UNTIL shot at. This encourages you to actually get shot by jumping in front of the survivors to hit you.
This may be overpowered, so to balance it, maybe you have to actually kill it for the pack of hounds to appear, or for it to be really slow, maybe with a missing leg.