Newfoundlands

Hey guys, I bought a newfoundland right about a year ago. Yes it was from a petstore (i know thats bad but i felt sorry for her). I have two other dogs, both older. About a month ago, she attacked my lab mix, twice, which resulted in a trip to the vet for her. My lab cowers whenever she is around now and we keep them separate (one inside, one outside) but anytime she sees him, even through glass, she will growl and snarl and if you reprimand her she just gets worse. When I got her everything I read said they were good-natured and got along with other animals. She is perfectly fine with my other dog. Anybody else had a problem similar or have advice to help her out? Thanks- Angela

Unfortunately, your dog probably came from a mill. There is no telling what her parent's disposition is, and how she was bred.
Have you tried to socialize her with other dogs, or taken her to obedience classes? What is she like with people?