When you rescue a large dog .
It is wise to know it's history as it could become a danger to you and other pets that you have in the house. You could use a trainner,but he or she will have a big job correcting bad habits that he may have aquired .If your dog is weak or (not well) a large dog by instinct will seek to harm her . so I might suggest that you take the dog back and wait to do your good deed of helping some poor maltreated animal when you can give it all your attention and even then it is a lot of work to correct the wrong that has been done to the poor animal . Only when with a litle guidance from a trainer or behavior specalist will you succeed. I hope this will help . Yours truly, Hugo ....