Did the chihuahua evolve into the mastiff or did the mastiff evolve into the chihuahua?
No, the question was, were both the mastiff and the chihuahua created in their present form with no changes whatsoever from the time they were created and now?
Actually, they both evolved from Wolves.
It was through breeding and conditioning, but evolved none the less.
(And techincally, I believe wolves evolved from Cynodictis which had actually evolved from Miacis)