Very Aggressive Dog!

Question
I have had many dogs over the years and have loved all of them very much...but the one I have now really stumps me with his behavior.  I have (Regin)...Female, about 5 years old..she is fixed...She is what some people call a bird dog, and has a very friendly disposition, but just mainly wants to be loved and is very laid back in temperament... My other dog (Zack)...Male, about 6 years old, is also fixed...He is what some people have told me is just like a Walker Hound.. I guess that is some type of hunting dog.  Short hair, and runs like the wind.   Zack has always had a grouchy disposition....He growls at anything, even when he is being petted.  My husband has recently passed away, and Zack has become very territorial.... he has always been that way, but now it is to extent, that if things don't go exactly his way, he will fight with Regin for no reason at all... just because he wants to, or Regin does something that Zack don't like.  Zack is extremely jealous, and has to be the center of attention.  He is a great watch dog, so I pity the person who would ever try to get into my house without permission.  If I let Regin and Zack out the back door to the yard.. Zack will push Regin out of the way just to get out the door first.... It is that type of behavior that bothers me.  Regin don't mind, but sometimes he will growl right back at Zack, even though he steps aside to let him out the door first.   Zack and Regin have gotten into some massive fights and he has drawn blood on Regin.  It gets so  bad, that even stepping inbetween both of them, sometimes does not get them off of one another.   Today, my daughter brought over her puppy... Female, Black Lab.. about 3 months old.  Sierra is a sweetheart and just wants to play.  Zack just barely tolerates the dog, and is constantly growling very hard even when Sierra approaches him.  I am constantly on my guard around them, because I don't want to puppy to get hurt.  I find myself constantly yelling at Zack.  Zack does know that he is wrong, but sometimes I think that "he can't help himself".... I don't know what he is thinking.  Well today, I was laying on the bed.. and Sierra.. the puppy came prancing in and jumped on my bed to say hello to me.  Zack was on the floor... He immediately started to growl, and before I knew it.. attacked the puppy Sierra.. He grabbed onto Sierra, and threw her around like she was a rag doll.  I got inbetween them immediately, but Zack had already drew blood on Sierra... a couple of teeth marks on her nose and neck.. Nothing serious. but it was still scary.  I love all my dogs... but cannot seem to break Zack of his aggressive behavior.. I don't know what to do, and get very uneasy when there are other dogs around, since Zack is so jealous.  I pay attention to him, and tell him it is "alright".... he seems to crave my attention but he won't let any other dog get my attention.  If I pet Regin, or pay a little more attention to her, Zack lets me know it.. and is right inbetween my arm, trying to prevent me from even petting Regin.
I don't want to get rid of Zack.  I love him so much, but someday I want to get a little lapdog.... A dog that I can take in the car, and just have by my side.  I don't think that will ever happen since Zack is so aggressive.
My son....who has 2 labs.. a yellow and black.. one male and one female.... and just recently brought in 2 Chihuahuas..(sp  ??)  They all get along just great.  The little dogs climb all over the big dogs and they are one big happy family.  What the heck happened to mine.  I see how other dogs behave, and wish I had the same thing.
I hope you can help me with this problem.   I am at my wits end.  I was going to purchase a muzzle.. but don't know if that is the right thing to do.  Will Zack learn... or is he to far gone and set in his ways to learn anything.
Thank you,
Barbara Erdman

Answer
First, I would recommend that you contact your vet and explain the issues you're having with Zack, and see if the vet thinks there might be some sort of imbalance or health problem that may be attributing to the problem.

Once any health issues are ruled out, then I would recommend that you begin "Nothing in Life is Free (NILIF)" with both dogs. You can find lots of information on NILIF on the internet by going to yahoo.com or google.com and doing a search for it. The problem is that you are not the alpha, or leader, so Zack does not respect you. Dogs do not feel jealousy, but Zack believes that HE belongs in the leadership position because you are not assuming that role.

I highly recommend that you consult a local behaviorist/trainer, because it's extremely difficult to help solve aggression issues over the internet.

I would also recommend that you NOT allow other people to bring their dogs to your house, or if you DO allow them to, put Zack away so he cannot get to them.