The giant rat bone is an item on the Odd Old Man's list during the Rag and Bone Man quest. It is dropped by giant rats and giant crypt rats. After the quest is completed you can no longer obtain this as a drop. The bone must be put into a pot of vinegar and boiled to create a polished giant rat bone before giving it to the Odd Old Man.
Monster Combat level Quantity Rarity Giant rat 3; 6; 26 1 Common Giant crypt rat 76 1 Common