Jack Russels, and Rat Terriers will instinctively tree a squirrel and stay under the tree it went up. All you have to do is spot it and shoot it.
There is also a couple of breeds call the mountain cur and mountain feist that were specifically bred for squirrel hunting.
Little terriers are fearless. A couple family in South Dakota had 2 Jack Russells. One day they let them outside and the dogs were going crazy at the bottom of a tree. The man thought "darn, they have treed the neighbor's cat again" He looked up and saw a 160# cougar.