I second the Jack Russell or the Jagdterrier as both are SUPER TOUGH smaller dogs that will run anything and wont quit. Both breeds are very smart and could easily be taught to retrieve. Many are used in tracking already as their small size and big dog mentality carries them through ANYTHING. Plenty of drive to chase squirrels and coons but as pointed out would probably run silent once they put the prey in a tree.

Check out the hunting pictures, they are running cats, hogs, bears, coons, even bringing in a mallard. What more can you ask for?