Yep, this actually happened. A family from China had to give up their 'dog' after it turned out to be an Asiatic black bear!!! It was supposed to be a Tibetan Mastiff puppy and the family was shocked it was eating a box of fruit and two buckets of noodles per day.
The family finally realized the dog was a bear after it grew to 250 pounds! It was even walking on its hind legs! But don't worry, the bear has been relocated to a wildlife center.