The term 'OLD HORSE' can be broken down into two crossword solutions: NAG (3 letters) and PLUG (4 letters). A NAG is an old horse, usually used for riding or pulling a cart. It is often seen as a symbol of hard work and perseverance. A PLUG is an old horse that has been retired from service and is no longer used for riding or pulling a cart. It is usually kept in a pasture or stable and is used for breeding or as a pet.