There is an underground corporation that sells exotic pets. At least...That's what it says on the brochure. The pets are expensive and there are two kinds. One cheaper than the other. You have your trained pet for the Man or Woman who doesn't have the time to do this themselves...These pets cost 15K each and then there are the Pets in Training that cost only 7K but come with a 'training your new pet' video. The price is steep, but for someone with a lot of money it might seem intriguing. When the so called pet gets delivered Well...it's a human girl in a box. If you bought trained she'll come with a red bow tied around her neck. If you bough unchained it'll be an electric collar. Both come with pet beds a leash and a water bowl.
Alternatively you could play one of the men/women who kidnaps and trains a girl to be a pet.
There are quite a few ways this can be played out. There can be a couple buying a pet, man buying for his girlfriend/wife or woman buying for girlfriend/wife. A lonely person just wanting the companionship a present from a friend etc. Maybe he/she thinks it'll be some special breed of animal and is actually surprised to find a human.