I really need help very important?

What your dog is probably experiencing is aggressive fear. They get so scared that someone is going to harm them that they lash out with aggression, like your chihuahua. Many small dogs have this complex due to their small size. They are sometimes mishandled by strangers and expect that when new people come around. Because your dog is from the humane society, it may be difficult to tell where this behavior arose from, seeing as that we don't know his past. Maybe he/she was abused or neglected and doesn't know how to interact with strangers. Or maybe he has the Napoleon complex, where small dogs think that they are really tough so they act tough as a front.

You should REALLY watch the show "The Dog Whisperer" with Cesar Milan on National Geographic. He has great techniques on how to deal with these aggression issues among others. To be precarious, you should get the professional advice of a trainer or dog handler at all times.

The very first thing that Cesar advises is that primarily you exercise your dog (no matter the size) for it to release aggression or pent up energy. Just because your dog is a toy breed doesn't mean that it doesn't need exercise. You accomplish this by taking him/her on short walks (~30 minutes or so). Dogs are a pack animal, and make important bonds with their owners/pack members during walks which lets them know that you all are a family and respect each other.

The next advice from Cesar is to discipline your dog. Not punish...discipline. Many people feel that if they scold a dog that they are being "mean," but all that you are really doing is acting like a parent. Would you allow your 3 year old to bite strangers without bringing it to their attention and let them know that this behavior is unacceptable by your standards. Remember: YOU are the pack leader, not your dog. Whenever your dog starts to even SHOW signs of aggression towards someone like pulling back it's ears, baring its teeth, or slouching in a stoic position, IMMEDIATELY correct them with a firm (not hard) touch to the neck area and retreat your hand. While you do this, make a noise that will distract the dog from its current mind set (which is "I'm going to bite this person if they don't leave me alone"). Personally, with my dogs, I make this sound "8!" or "hey!" clearly and confidently - not angrily. Cesar uses the sound "TSSST!" When the dog hears and feels this correction, he should stop.

This will take lots of practice though. It won't be fixed the first time around. It is crucial that you perform this correction when you witness the "signs" of aggression building up. Not during or after. Do NOT in any way, form, or fashion cuddle, pet, stroke, or even touch your dog affectionately after he bites someone or shows aggression - this is positive reinforcement, telling the dog that when he/she bites or is aggressive, they get pet and hugged. You might just be trying to tell the dog "It's OK, baby. Don't bite anyone. Why did you bite them?" in an attempt to comfort them, but all they see is: I bite = I get pet. You are essentially nurturing this unstable and aggressive behavior.

Affection is the last thing on the list. Affection is necessary for most creatures, but only when given at the RIGHT TIME! Don't show affection to your dog when he/she does something you don't like. Give it to them when they are behaving, when they DON'T bite, when they get back from a walk with you and have behaved, when they wait for their food to be served, etc.

Go to Cesar's website:

http://www.cesarmillaninc.com/

He has great tips on how to walk your dog and much more professional advice. Hey, he might even be able to visit you and put you on his TV show to help you with your problem!!! I advise that you try contacting him or his crew so he can help you personally. Let me know if this is helpful in anyway and if you get on the show!

Get a trainer. You aren't going to fix an extreme case of aggression on Yahoo Answers

I think your best bet is to get a trainer, they will beable to see the dog first hand and understand the underlying issues. It may cost some money now, but save you money down the line and probably his life

You might check with the place you got him and let them know what is going on. They may be able to put you in contact with a aggressive behavior trainer.

Sounds like fear aggression.. Dog with no confidence, not properly socialized and no training.. Sounds like a good time to sign up to some training classes once the dog has been properly evaluated..

Get a trainer. Whatever you've tried isn't working, and unless you want a lawsuit in the near future, a trainer is your best bet.

A muzzle or cage whenever you have company is the first step. Then training will help. He is afraid and you need to be the dominant leader.