Blue heeler/Australian cattle dog. Mine has killed a roof rat.
They are exceptionally loyal and intelligent. They love having a job to do. If you train to get a squirrel, they will do that job with utmost passion.
They are very athletic, fast, agile. I would imagine one of few breeds that could outmaneuver a squirrel.
I absolutely adore this breed. Only thing about them, if you want them to get along with other pets and young kids, they have to be introduced to them as a puppy.