[yorkietalkjilly]

Your rescue may be a resource guarder or just new and trying to establish his position in his new pack. Likely the two will work it out over time though some never do but this is kind of to be expected when bringing a new dog into home. I would keep them separated when toys, chewies, food and intense play or a lot of loving is going on for the time being while working with the new dog as outlined below. You might just ask the other dog to leave or take it out of the room, let it outside and out of hearing range if possible while the other dog is playing with toys, eating or having loving sessions and getting attention from you for a while as things settle down.

Here are some more things you might consider to see if they might work for your situation. Start the Nothing In Life Is Free program(Google the term for several sites with info on how to do it) and a good obedience training program with the newbie, to slowly teach him that you are his new pack leader and that learning to do what you say and respect you as the authority figure in the pack is the only thing that is going to make him the happiest. Training a dog, the bonding and trust that is formed during nothing but positive reinforcement of carrying out commands, can help to settle a dog, give him work to do, keep his brain busy learning and challenge him to learn impulse control - something he'll need to live well with his pack mate.

Lots of exercise, long, productive walks after a good sniffing session up the block so he can "read the news" of visiting critters outside and some interactive toys can also help to take the nerves and insecurity from a dog who is trying to fit into a new pack. Later, after your newbie has some impulse control and respects you as his sole pack leader, you can teach him the "Leave it" command and use that whenever he is showing unwanted interest in something he shouldn't - such as his new pack mate or a piece of food that got dropped and both dogs are going for it, likely producing a fight.

Then, after he's learned the "Leave it", you can give him that command when he is resource guarding, toss meat treats across the room from what he's guarding and when he relinquishes his trophy and runs for the treat, give him a second treat for "Good Leave It". Kind of bait and switch that will teach him that every time he gives way on something he wants when you give that command, he gets two great treats and lots of praise. Keep all training fun, upbeat, loving, gentle & free of discipline and just use the word "uh oh" when he doesn't respond or doesn't get his trick right during training. Later when he knows better, has learned "Leave It" and knows that command well enough to do it even when he doesn't want to 99% of the time, the 1% he doesn't, say "No" and walk up and shoo him away from what he is guarding. Place him in another room with the door closed and leave him there around 15 minutes, no matter how he screams and barks. That will eventually teach him that disobeying and hearing a "No" will get him quickly removed from the area of family gathering and activity and leave him all alone for a time. Then, when you have to resort to "No", he'll start to back off that 1% before you have to isolate him.

A good behaviorist is probably best but if you wanted to try some of the above things first, it might be worth a try.