It is an interesting situation. It's not your dog, for one thing. You can't really take the liberties with this dog that you take with your own dog. But I do appreciate you're trying to get him some exercise and work with him, knowing he might be yours someday. I would just say, "Warning, Will Robinson!" there are a lot of potential problems here. Do you have any experience working with a difficult dog at all? Frankly, I am worried about YOU and OTHERS potentially getting hurt. Dogs who growl often back up their warning with a bite if things don't go to their liking. You could be trying to get the muzzle on the dog when the dog decides he doesn't quite like that idea, with you, who he's not so sure about, and he solves the problem the best way he knows how, and now you're dealing with a bite. Do you by any chance have a fenced in yard? I would probably start by playing with him there and starting to build a relationship, first. See if he'll fetch a ball. At age nine, he's not a spring chicken and he may not be able to do much, but see. Part of his problem *could be* that he's in pain. If that's the case, he may need to be on pain medication first, before you do anything. (Which, of course, requires a health eval.) If he responds positively to play in the yard, perhaps you could begin venturing out for walks. But be very cautious. It sounds like you don't know this dog very well and don't know what his triggers are. You don't want him to bite anyone, or jump on anyone, etc. Be very careful about letting people come up and pat him, you just don't know how he'll behave. I don't have a huge issue with a muzzle, but it won't necessarily solve all your problems. If a dog hurts someone by jumping on them and knocking them down, you're just as liable as if the dog bites someone. And you could potentially be liable even if it's not your dog, if you're in charge of the dog. Not saying this to scare you or to change your mind about taking the dog out, but I am saying there are a lot of different issues here at play. It can be a difficult challenge taking on a dog that's never had any training or socialization at all.