I knew a lady once whose Belgian sheepdog underwent a similar personality change in the last year of his life. She couldn't have peace of mind without knowing why her beloved companion for so many years became an aggressive stranger, so she had him autopsied and they found a large brain tumor.

On the other hand, a victim of abuse may lash out once he is in a secure setting. Post traumatic stress syndrome affects animals as well as humans.

Could be a brain tumor, could have been heart trouble could have been a parasite he picked up while on the streets....So sorry you and your friends had so little time with him...Fo the future you or someone you know has this kind of thing going on, take the pet to the vet ASAP dont wait and always try and have a necropsy done - that way the owners can prove the animal was not acting appropriately due to a medical condition....

Mazar, that's what I thought. I have heard this many times with brain tumors.

So sad for the family.

So sad for you and the family. You were given good advise here, things I didn't know that I'll mentally file away for future reference.

So sorry to hear this. Did your friend bring her dog to the vet to find out what was wrong? Sometimes a low thyroid is the problem, some dogs become aggressive but the meds usually help them. My neighbours GS was getting old and became aggressive. She took him in for testing and he did have a brain tumor.

I also knew someone whose sweet dog suddenly became aggressive due to a brain tumor.

They can also suffer from mental illness just like a human. People that have severe mental problems and commit horrible crimes aren't that way their entire lives - sometimes the disease progresses slowly and sometimes fast. There is a million reasons an animal can suddenly change personality and be dangerous.