Why is my 9yr old staff loseing weight at her back end

Does she walk the same and run the same as always? How much food is she eating and when was the last vet appointment? As dogs age its fairly common to see "smallness" in the rear hips but 9 is not that old.

YOur dog could be suffering from muscle atrophy. Common in dogs who suffer from arthritis. Have you spoken to a vet about your dogs weight loss? There are home treatments to help relieve some of the pain associated with arthritis.