Unused baked potatoes
I have done this in the past and it does work well (think about twice baked potatoes...) but there are few considerations. You will need to add more butter or cream to the baked potatoes and it does work better if you are mixing it with boiled potatoes. I don't recall there ever being enough left over bakers that we didn't need to add at least some fresh potatoes. 



bonus: then you can use the left over mash potatoes to make potato rolls for the next night :)