Mixed with Yorkie and ????? I was told Shih Tzu. He has a under bite also. What do you think?

I don't see any Shih Tzu in this puppy, but possibly Yorkie. Your guess is as good as the guess of anyone. Besides that, if either it's Sire or Dam or both weren't purebred dogs, then there could be a dozen of different breeds in this puppy's background. It looks like it's going to be a fairly small dog when it's an adult, but you've asked this question multiple times now, and no matter how many answers you get, none of them can possibly be accurate.....they are ONLY guesses. This is a mixed breed puppy of unknown origin. Be satisfied with that, or just call it a Yorkie mix, mutt or mongrel.

Me too - No Shih Tzu and perhaps some Yorkie. When you buy into mix-breeding, you are buying into a lottery, much as when he gets a bit older, what he has in him may be more obvious.

Yet another guess on top of all the other answers you have had to this question ....... if he has an underbite already, keep an eye on what's going on because fact is the lower jaw keeps growing after the upper jaw stops, so he could end up with a problem, not just eating, but how his teeth wear.

Add - Looking at your other questions/photos, I don't really see much Yorkie in your puppy - if any.

He appears to have the Yorkshire Terrier colouring [but then lots of breeds have that sort of colouring so it could be a mixture of any breeds] and I can also see the potential of Shih Tzu; but as others have said, he has quite a short coat which isn't common in Shih Tzus or Yorkshire Terriers. Those two breeds could be in there but it looks like there's probably some other type of Terrier.

Why does it matter as long as you're caring for it properly and love it.

both yorkie and shih tzu are fluffy dogs. So there needs to be a short haired dog in this dogs ancestry.

From this angle it is hard to tell, only the face to go by, but maybe some small short haired terrier might be in the mix.