over-protective Aussie

Question
We have a beautiful solid red Australian Sheperd named Chewy. This is Chewy's 3rd home in under a year. My neighbor's brother gave her Chewy because he was too energetic and chewed everything up...hence his name! My neighbor was also unable to tolerate his puppyish ways and let him loose in the neighborhood so as to avaid paying the Humane Society. Awful, I know. Well, we took Chewy in with the intentions of finding him a home, which we did...OURS! He's an amazing dog, he's just VERY protective of his family. He doesn't trust new people or like to let them in the house. He's never shown any signs of aggression towards my husband or I nor any of the kids, but he does growl and bark like crazy at anyone entering the house or yard. We have 2 other dogs besides him, a pomeranian and a black lab. He and the black lab are trying to establish whos the leader between the two but they are starting to iron out the rough spots between them. He's very careful as not to hurt either of them when they play, he's actually the one that always gets hurt. He's very sweet and affectionate and smarter than heck, but he scares the crap out of everyone that enters our house. Please help!!

Answer
It's not uncommon for an Australian Shepherd to develop an aggression problem; I've seen many.

This is a dominant, smart, energetic and very controlling breed. This  particular dog has no doubt received absolutely NO socialization, and you are most likely seeing a combination of control and fear (in terms of visitors and "strangers".)  You may also be seeing the result of cruel or abusive treatment.  When any actively aggressive behavior is demonstrated by a dog, that's an indication that you need the in person, experienced evaluation by a certified applied animal behavior expert.  If you are in Britain, you can contact the Association of Pet Behavior Counselors.  If you are in the US, you can call the veterinary college in your geographical area and ask for a referral to a Ph.D. who can come to your home (because this behavior needs to be observed in its home environment.) It appears that you haven't had Chewy very long (because he and your other dog are still working out rank issues), so your window of opportunity to address this stranger aggression (which can escalate into a visitor being bitten and you being sued) is good; but it must be done carefully and under the supervision of an experienced professional (NOT a dog trainer!)  I also suggest you change the dog's name; conditioned response to sounds (and that includes a name) can be strong.  Begin using the new name (so long as it doesn't sound anything like "NO") while introducing positive reinforcement training.  Train one strong behavior ("sit", but use a made up word) until the dog offers a 100% successful response to the command.  This enhances the dog's ability to problem solve and promotes you psychologically, thereby giving him less need to control your environment.