You can make a cute little sweater for your dog in just a few minutes without sewing a stitch. Start with a old shirt. I picked this up a the thrift store. I looked for a shirt large enough so that the sleeve would go around my dog. She weighs 6 pounds so I uses a boys medium. I have used felted sweaters for a even warmer sweater.
Start by cutting off a sleeve at the shoulder.
Measure your dog and estimate how long it needs to be, then cut off the cuff end. Take in mind how much stretch there is to go over the head and fabric for a folded over cuff.
Fold the end over a couple of inches.
Cut two slits on either side of the seam that will be under the dog. My dog has about 1 inch between her front legs so I try to keep about 1 inch intact. The slits should be about 2 inches long.
Slip over the head and pull legs though slits. The natural cut of the sleeve works out just right for the back side, mine anyway as she is a female. You might cut it in more for a male.