This is aggression, right? Please help!

[Dracoluna]

I agree that this sounds like a very insecure dog. I've trained several rescue dogs with this issue and honestly, the first thing I've done with every single one is to grab them the second they growl/snap by the neck and pin them to the floor (only if you can physically handle it though with that small of a dog, shouldn't be a problem). It doesn't hurt them (obviously you do not choke the dog or even put much force behind it) but it tells them in their language that that behavior is unacceptable and that you are the pack leader. You can also scruff him if that's easier to do as it's less risk of getting bitten. Since the dog is doing this just with being unsocialized and no physical abuse, it signals that he is a more dominant dog. He's trying to be boss and that needs to stop or someone will be bitten. Each time he gets away with it, it reinforces the behavior. Many times, flipping and pinning them on their back by the neck only needs to be done once to get the point across. You hold them until they lay there calmly and accept the pack hierarchy. After that, then you can start working with him if you wish.

Honestly, a dog with that sort of reaction is going to take a strong owner and if taken to a shelter, very few are going to bother trying to rehabilitate him. Your roommate needs to realize that and quickly or she may well get bitten as well. It doesn't matter how small the dog is, they can do a lot of damage. I had to train a schipperke that had been abused and resorted to biting to protect herself. She'd torn a gash in the guy's arm that needed over 20 stitches. I flipped her once the first time she tried to bite me, and she was a great dog after that (my brother's dog). She still needed guidance and would test the boundaries a bit, but she never did bite another person. Same with my miniature poodle who was simply unsocialized as a puppy. He tried once, got flipped, and became a lot more stable dog after that. The fear goes away because they then realize where they stand in the pack.