We just make up stories of where they come from. Toki, my older dog (hound mix), his story we made up is that he was an unbelievably cute puppy (seriously, he would have been). Previous owners fawned over him.. but then he grew.. and grew and grew. He wasn't neutered or trained, and after he took off jumping fences and chasing animals, they were just relieved that 85lb menace was gone. Had him for 11 years now. Worst-best dog ever.

Our other dog, Olive (border collie mix), we knew had a nearly identical sister in the shelter and they had to separate them because one would get protective and aggressive with the other. The other sister was already adopted when we got there. Olive was terrified of everything, scraggly, underweight, and had pneumonia from kennel cough. She had no idea what a toy was. Her story we made up is she was born in a rural area. The litter didn't get much attention. She and her sister were dropped off somewhere, and they lived at the dump, scavenging food (she was horrible at getting into trash and scavenging wherever she could when we got her). She is terrified of garbage trucks, we say, because they would come through the dump and scare the daylights out of her.

That's how we handle the mystery of our adopted animals' previous life. Just make it up based on their behavior, appearance, and habits!