The best thing to put in a dog house for warmth is an insulated pet bed. Insulated pet beds are designed to help keep your pup warm by trapping body heat inside and preventing cold air from coming in. Additionally, you could also use a heated pet bed or place blankets over the top of the regular pet bed. To ensure maximum comfort and warmth, consider adding a layer of foam padding as well. Finally, do not forget about ventilation; make sure that there are openings for fresh air circulation so that your pup does not get too hot or stuffy!