What should I do? I just rescued a dog.?

Take the dog back to the shelter.

He is way too much for you to handle and it sounds like your other little dog "could" be in danger.

The dog has obviously got a dominant temperament and if he like this after only 3 days, he will most likely be even worse after 3 weeks.

You have a volatile mix of dog their that needs very experienced handler to make sure the dog respects them.

The dog may be best suited to a home with no other dogs to compete with.

Your choice though.

But just to say, do not allow sentiment to cloud your judgement.

The dog has no real bond with you yet, and quite honestly could not give a damn about you. He will quite happily trot off back to the shelter and hopefully get a new owner that has no other dogs. Jmo.

Keep the dogs separated. Call the shelter and see if they have any counselors to help advise you, in this situation. Somebody there certainly knows this dog very well.

Was he fostered or tested around other dogs before placement with you? How much did they KNOW about his background? Was he ever used for dog fighting (or does ANYONE even KNOW)?

How thoroughly did they go over YOUR HOME SITUATION and the fact that YOU HAD ANOTHER dog?

Did you know -it is NEVER recommended for you to get another dog that WEIGHS MORE than 25 lbs more than your current dog? Obviously, neither you nor they obeyed this common sense RULE in adoption.

Even if he had not shown aggression or potential resource guarding - he could EASILY HURT her - if he weighs more than 25 pounds more than she does ==>>> by simply BODY SLAMMING HER! and for that reason alone (IMO) he should go back.

Wake up and smell the coffee. Return him AT ONCE. This is NOT a safe dog for HOMES WITH OTHER DOGS. He is beginning to feel comfortable and is beginning to show HIS TRUE SELF which is....... dog aggressive.

He is part pit bull. ALL they are USED FOR IS DOG FIGHTING. None are responsibly bred. Rotties are dog aggressive and very protective. That is a LOUSY mix. He is TOO HIGH energy and is spending too much time crated. You are not in a position to let him out because of the lack of safety to your OTHER dog.

He is also not getting NEARLY enough aerobic exercise. And that is UNLIKELY to change anytime soon. He lacks even basic LEASH training and darts AWAY FROM YOU!!! -Which is a BAD sign and unsafe, from multiple perspectives.

Have you even checked your home owner's insurance? Many will NOT COVER YOU (and will DROP YOU) if you own EITHER of the breeds he is comprised of! I do not care how upsetting it is, TAKE HIM BACK because regardless of what happens to HIM, if he injures or KILLS YOUR DOG - you will NEVER get over it. Not all dogs can be saved or OUGHT TO BE SAVED especially at the expense of good pets like the one you already HAVE.

The thing that people don't realize about rescue dogs is that NO ONE, even the shelter staff have any idea what the dog is like in a home and one on one situation with people or other animals. The dogs just act differently when in a shelter and once in a home they become comfortable and any bad habits they have come back. This is something I hear time and time again is that after a few days the dog becomes aggressive or has food aggression. One reason that I will not adopt a dog if my grand kids are around or I have other pets. I would keep them separated unless the large dog is on a leash and if this behavior continues or seems to escalate, take him back. There is always a reason people relinquish their pets.

During training either the leash is on, or the dog is in the crate. 24-7 no exceptions. This seems harsh, but you must establish control early on especially with a strong breed.