Dog on Dog Aggression - My dog just bit my puppy for no good reason

My one and a half year old Yorkie Chihuahua Mix has just bitten my 13 week old cavalier King Charles Yorkshire Terrier Mix bitch on her face, he has NEVER bitten anyone or anything but can get a little territorial so when introducing them yesterday after bringing her home, I moved all his belongings he would get grumpy about although he was very grumpy and had a few grumbles at her yesterday, they slept in the same room fine last night and today when my Chihuahua Yorkshire Terrier Mix was in his bed, she walked past and he lunged at her, my partner screamed in shock and he let go after a few seconds. She has a small cut on her head which is not deep, but she is now petrified and I have no idea how to rectify this behavior, I have left him in a room alone for half an hour and I am hoping he will learn that when he is nasty to her he doesn't get to sit with us, and when he's nice to her he gets rewarded but i'm so nervous he will be like this forever now hes shown this bad behavior we have never seen from him before.

Hi and thanks for your question this is a problem that needs to be sorted fast - I will give you some advice below but I would get a professional trainer in to observe the behavior and correct it as it happens.

Below are some tips...

1. Dogs have to work out who is the leader of the pack - dogs don't have to be at the top of the pack or low in the pecking order (they don't actually care) they just need to know that they have a social structure and a place in it (wherever it is).
2. Watch your dogs interact with each other - your older dog is currently trying to assert his dominance and his place in the pack.
3. If you do catch your dog in the act say 'No' and separate them - introduce them and reward your older dog when he is well behaved.
4. Reinforce your older dog's position in the pack so he knows he is still top dog - this means feeding him first, let him go through the door before your other dog - the alpha dog will always lead in most situations - this includes body language.
5. Because your dog has actually drawn blood I would advise you to get a canine behavior expert to observe your dogs together to determine the best course of action.

Hope this helps

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