A hardwood floor’s finish is designed to protect it from unwanted moisture, but pet stains are trickier to clean. Even if pets have never wet the floor, they may think they have to mark over pet stains from previous households. Pets will also return to their own previously “marked” areas.
If you get there in time, you can blot the stain with paper towels and rinse the area with white vinegar. Follow with a pet stain cleanser (specifically formulated for hardwood floors) and blot up the excess.
Most stains can be removed by scrubbing or sanding the wood to remove the protective finish, (the sheen). Start with #000 steel wool and wax. Once the stain area is stripped, you can apply mineral spirits to cut the grease or oil within the stain, and then rub it dry with a soft cloth. You may have to retreat with bleach or vinegar, and soak the spot, rinse, apply a dry cloth, re-sand, stain, wax and hand-buff.
Older pet stains may have seeped through the wood and into the subflooring, necessitating removal and replacement of the wood planks and subflooring. You’ll have to stain and finish new wood to match the rest of the floor, which could possibly involve hiring a professional.
Other remedies are also tried and true, including hydrogen peroxide, ammonia, and bleach that you can use as cleansing agents. Just be careful when you are mixing cleaning agents with water not to over-soak the wood.