The term 'Littlest Pups' is often used to refer to RUNTS, which are animals that are born smaller than their littermates. RUNTS are usually the smallest and weakest of the litter, and may have difficulty competing for food and resources. They are often more vulnerable to disease and may not survive as long as their larger siblings. Despite their small size, RUNTS can still be healthy and happy animals, and can even grow to be larger than their littermates.