Every wood flooring manufacturer mentions pets in their maintenance instructions (we do too). The recommendation is to keep their nails clipped so as not to leave any potential scratches or gouging in the floor. Our 55-pound poodle can leave occasional evidence in our American Cherry, which is the softest hardwood we carry. It only happens when the neighborhood kids come over and throw the ball down the winding Cherry staircase, and the dog slips. Our Maple floors, which are substantially harder than Cherry, survive anything she can dish out. The real big and heavy dogs (i.e. Saint Bernards, Newfoundlands) usually don’t move around fast enough to leave marks.