Adopted chihuahua -possible abuse case

Question
I adopted a 9 yo male chihuahua on Friday. He was terrified at the shelter but once you got him in your arms, he was calm. I decided to bring him home and he did wonderful in the car. Now it is day 3 and his main issues are aggression (warning bites) when trying to pet him or pick him up along with yelps. He takes treats from my hand gently and responds happily to the words treat and outside.  I would like advice on trust gaining with him. Also, he wears a harness fine, but when I bring him outside on the leash, he yelps horribly like it is hurting. He seems immediately terrified with the leash. Is it possible it was used against him before? How do I slowly train him to not see the leash with me holding it as a threat? (side note: i have a 3 yo chihuahua, so it's not my first rodeo with the breed.)

Answer
You may be right on having been abused. Or the other scenario is that his previous owners just let him get away with everything, which lead him to the shelter. As you know, they are a pain in the butt breed. The main thing to remember is that whatever new or any situation comes along and he freaks out, it's up to you to remain calm like it's no big deal.

There's actually a few things you can do, but remain calm and positive the entire time. Since the leash is a big deal to him, I would start with just laying it in plain sight and let him sniff it likes it's just part of the house. The next step in the process is putting it on him just around the house so he gets used to the feel of it on him and he can realize it's not out to kill him. Regardless if this is a fear case or simply no manners, these processes with both make him realize everything it not out to get him and it teaches him that, "guess what, things are going to happen, you will be able to deal with it and trust me, you'll have fun!"

The next step in the leash process is the hard part, and that is letting him freak out and cry until he stops and calms down. And all you're going to do is stand there and do absolutely nothing. His reward for this is going to be having fun going on walks!

Now for the aggression/yelping when being picked up. If you are able to do so without getting bit, ignore his behavior (as he may be doing this to get his way) and pick him up anway.....only if you are positive you are not going to get bit!!!! If you succeed with this already, reward with a treat once he's in your arms to associate with a positive experience. Or you can simply walk around like it's not big deal.

The other way is more difficult to explain, but it involves him being comfortable with the leash being on him and using the leash as a pull-away guide to pull his head away from you, then picking him up. This sound confusing, so I'm hoping you'll be able to pick up without having to do this.

Something on the side I want you to start implementing is "earn and reward." This basically means that he must follow a command prior to eating, walking, attention, etc. This will both help boost his confidence if that's the case right now; as well as place you in the alpha role and him understanding his role in the family as well, in case his behavior is just bad manners.

Keep me posted with this and let me know what further questions I can answer.