I started working with a Chihuahua puppy who is a sweet, nice little dog. However her family noticed some trouble when they tried to put the collar on. This little dog seemed to become aggressive as soon as they put on the collar and stayed aggressive the whole time it remained on. They are not able to put a collar or harness on or use anything that has to do with a leash. They are not taking the puppy anywhere other then the vet and training because they can't put the dog on a leash. Otherwise the puppy seems friendly and was comfortable with me reaching down to give her a treat, walking around the room etc. She is a sensitive dog and a bit nervous when she first arrived but quickly got used to the environment. The vet couldn't find anything wrong and didn't think it had anything to do with pain. Week one was owner sitting with their puppy in a puppy class and giving their dog treats when they saw the collar, treats for trying to lure their puppy through the collar and generally getting them used to the collar. Week 2 was a private lesson. Letting the puppy explore the room and spending about 30 minutes getting the puppy comfortable around a leash on the ground, jumping over the leash luring the puppy's head through a looped extremely light home made leash. Then we got the puppy to put it on and comfortable with it going on...but she had trouble with me taking it off and started snapping so we ended up just draping the leash over the puppies neck and giving treats for leaving it on and walking with it draped over her neck. I sent them home with the leash I had had specially made for them. (Crocheted with a thick thread and tiny light plastic hook with just about no weight to it.) So my questions are is this normal for such a young puppy? I know that Chihuahuas are prone to fear and aggression, but I have mostly worked with older dogs. Any suggestions on what to do, things to watch for, avoid etc.?