I don't have a pattern, but if you do have a tiny chihuahua, I have a little bitty sweater I made before picking up my puppy you may have if you'd like.
As you can see, it will fit a coke can sized puppy.
I made it up as I went
I'm not really big on patterns - I guess I don't have the patience to count and "follow the directions", so just figure it out as I go. You are welcome to have this one and make a pattern of it if you'd like, or I can take some pix of it laid out flat and send those to you.