Complex sentence: The protagonist Potter chose for the tale was an eastern grey squirrel, a species exported from America to Britain, but it was unlikely she saw the animals in the woods around Hill Top as they had yet to become established in the Lake District.
Split into simpler sentences: The protagonist Potter chose for the tale was an eastern grey squirrel. It was possible (but unlikely) Potter saw the animals in the woods around Hill Top as they had yet to become established in the Lake District.