@keytagnan
I actually think Shiro was originally his dog, one of them that was abused and probably killed by the son. The artist made sure to let us know that the business rarely saw customers, it probably wasn't even used. The dog fur was coming from the womans ghost.
She mentioned he abused animals and bullied children, but she used the term 'abused' when she mentioned how the son treated *her*. She also mentioned the old man hated people, but loved dogs...and then said he also loved her very much as well. If she was a person in life, he wouldn't have loved her.
The ending image also showed them both as dogs (with the dogs that had to be put down), with the old man finally becoming what he felt he truly should have been born as- a dog.
Also notice that there were 2 dogs waiting for the old man and Shiro- Shiro completes the trio! She was one of the 3 dogs that the boy used to kill the old woman.
Thats my theory, anyway!