KYS,

Yes, dogs do indeed mourn. When my beloved Lab/Shep X passed away 4 years ago, my remaining Lab went into a deep depression over the winter. She had no interest in walks or playing, just wanted to sleep. I got a new Lab puppy the next April, and she hated the new puppy for the first 2 weeks. She was meaning to really harm her. But after 2 weeks she seemed to snap out of it and they became, and still are, best friends.