If you believe the dog is being neglected or abused you can surrender it to a local shelter or humane society . You can take some photos of the dog beforehand and reach out to the craigslist post explaining you found the dog and surrendered it to X humane society or Y rescue . If the dog is indeed missing and his owners are looking for him the shelter will put a hold on the dog for a few days for the owner to come forward and collect the dog . If the dog is neglected and has medical issues the humane society will bring these issues up with the owners when they come to collect the dog . Shelters will not automatically euthanize a dog they evaluate them and make decisions based on temperament , health , etc .