Dog sweaters/tube dresses are not as complicated as I thought. I would crochet one in the round using double crochet. For a girl dog, this is easy enough to do. For the boys, you will have to switch over to rows towards the end so that he has the open area he needs underneath.
If you do the tube style, please make sure you make it big enough to go around the head and shoulderblades. If you want it to fit snug you will need a way to fasten it together. Measure your dog before you decide what size your dog is. Make sure the collar will go over the head and not just around the neck.