13 years ago we had a similar story. Son found a 4 week old behind Target in a field under a tarp. Mom was gone, so he brought home a little black ball of fur. We had no idea of what it was going to be. Wife was asleep from night shift work and he put the pup in the bed with her and she thought it was the daughters hamster!
She started growing and growing and turned into this.
Old girl is now on her last legs and her spine is eat up with arthritis and she can barely get around. Its a real chore to get her outside up and down the stairs for her to use the bathroom and we're spending about $75 a month on pain meds to make her a little more comfortable. Little shit has a routine of sneaking out and eating the cat food after I leave for work. She's so bold now she comes out of the bedroom and hovers around the area until I leave to eat it. I used to get pissed, but she's underweight, cat is fat. So I now fill that cat food bowl just before I leave so I know she's getting extra calories.