Dr. Dawn is correct – you must get the second dog neutered, and even then you need to keep these dogs apart (so they can’t even see each other!) for several weeks afterward.  This gives time for the testosterone to work its way out of the new neutered dog’s system.

Then, get the help of a professional behaviorist to reintroduce these two dogs – don’t attempt it yourself because they now have a history of aggression and will associate this aggression with each other.  You need someone that can read the tiny signs a dog gives when they are uncomfortable with another dog.  Even your own fear of something bad happening can trigger aggression between these two, so find a way to get help.  If nothing else, contact your local SPCA and tell them your situation – and ask if they have someone that can help you, maybe a volunteer or a trainer that will help at a reduced cost.  Explain that you are trying to avoid having to re-home one of the dogs and maybe they can help in some way.

Good luck – and don’t wait to get that boy neutered!   

They are almost a year old and have appointments scheduled to get fixed. You response is very insightful, but I do have one issue. One of them is a female. Does this make any difference? Thanks.

and i agree with PK, this NEEDS to be handled at home. anyone giving you further advice online is being reckless. you need someone in your home, watching your dogs interact. until this can happen, continue crating each dog separately and rotating which has freedom in the house. this will help you to keep them safe from each other. every fight will get worse, every interaction will escalate, and you may end up with a dead dog if you continue to allow them to be around each other.

Wire covers would work to protect the plant

This is how my rescue recommends you introduce a new dog to your home (and I do this with all fosters coming to live with my pack of 4 terriers, one of which is a Cairn/Chihuahua mix):

Before the new dog arrives pick a place in the home for him to live most of the time for 3 days.  I use my guest bathroom some times, and other times I use my craft room – both have tile floors so it is easy to clean up any accidents that may occur with the new dog.  Inside that room I place a dog crate appropriate in size for the new dog/puppy.  

For the first 3 days after the new dog arrives, we play musical crates and/or rooms.  I do not allow the dogs to see each other, they only smell each other on me, and under the door, etc.  When my dogs are loose, the new dog is in the prepared room.  When my dogs are crated or in their kennel the new dog is allowed to be out of the room, in the yard, or house with supervision.  There is ALWAYS a closed door between my dogs and the new dog.

I spend time playing and training the new dog each day, and will crate my boys for an hour or so in the evening so I can just plop on the sofa with the new dog for cuddles.  With a puppy you will need to be spending a lot of time with it as you will not be able to resist!  Spend an equal amount of time with the older dog.  One of the activities you want to focus on with the puppy is learning to walk on a leash – this is critical for the 4th day.

On the morning of the 4th day, put leashes on both dogs and immediately go out the door for a walk.  Don’t give the dogs time to sniff or eyeball each other – the job is to walk together with you for at least 10 mins.  If the puppy is older keep walking (5 mins. per month of age is a good rule of thumb – too long a walk puts too much stress on the bones and joints of a puppy).

Once we have finished the walk I take all the dogs into my fenced yard, drop the leashes and allow them to sniff, play, ignore each other as they see fit.  Dragging the leashes allows you to step on a leash or pull one dog away from the other if things get hairy.  

I have had 99% success with introducing dogs this way.  The only exception was a foster that decided my smallest dog was prey – good thing I had that leash to grab!  It saved my dog’s life.

We believe the reason it works is that it allows the dogs to smell each other without any misunderstandings of body language.  In the wild a lone wolf will spend weeks haunting a new pack’s territory.  They stay mostly out of sight, but scent mark in the territory.  Then they start showing themselves to the pack from a distance.  Finally they approach members of the pack.  If the pack wants them they are welcomed (usually by the female members of the pack).  So while our dogs are no longer wolves – smell is the first thing they pick up on.  First scent, then sight, then hearing.  So this 3 day of separation, but crossing each other’s scents helps the dogs get to know each other without confrontation.

With my dogs it works so well, they don’t even sniff each other’s butts once we are done the walk.

Since your dog is used to fighting through a fence I caution you to make sure that he and the new pup never see each other through a fence or crate during those 3 days.  Keep that solid, shut, door between them.

Your 13 year old dog may never want to play with the puppy – but the puppy will want to play with him!  Be sure your older dog has a place he can escape the puppy – such as a dog bed or crate, maybe in a different room.  When your Chi is getting too much attention from the pup, tell the puppy to “leave it” and ask him/her to play with you.  This will help the puppy learn to leave the older dog alone when he is in his quite place.

An Italian greyhound is a better size for your current dog, and they generally are ‘softer’ dogs.  The other 3 breeds you mention will be a real challenge for you, and will overwhelm the Chi.  It is a giant leap going from living with a Chi to living with these 3 other breeds.  They all are high energy, need WAY more exercise, and a lot of training to become good canine citizens.  The Border especially, will need a job.  Borders are scary intelligent.   

All that being said – give your dog a month with the new pup before you decide if it is going to work or not.  Good luck!