As a concerned citizen, you can report an alleged crime, but you can not prosecute a criminal case by yourself. Filing animal neglect charges is something that your local animal control or other prosecutor would do. I would start by reporting the conduct to animal control in your town or city.
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You asked this question slightly differently yesterday. Currently, the dog is not being mistreated and the the dog belongs to your roommate. If in fact that dog is mistreated after you leave, you can then report the abuse to animal control. Perhaps you can work out an agreement to purchase or adopt the dog from your roommate.
This is not to be construed as legal advice. I do not have an attorney client relationship with you.
If the dog is at risk for abuse (you'll have to show it if it was past but not present), contact the local humane society charged with prosecution of animal cruelty. Even if they find none, they might agree to keep an eye on the dog. You have no legal claim to the dog, however. Offer to buy the dog from him. If he doesn't care for it and you do, and he makes some money from it, you both make out.
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