We just got back from our 8th trip to Yellowstone which included starting in Northern Idaho and driving down through Montana. We were surprised at how much wildlife we saw in Idaho and had a great time in Yellowstone. We ended up seeing 23 mammal species including Columbian Ground Squirrel, Red fox kits, Yellow-pine Chipmunk, Gray Wolf, Pika (although not without a lot of looking), Beaver, Otter, and more.
We missed seeing Badger and Pine Marten although others we spoke with saw both (especially Badger) while we were there. Otherwise, we saw all other relatively common mammals except for Uinta Chipmunk.
Here is a link to the day by day report: http://focusedonnature.blogspot.com/2013/06/idaho-and-yellowstone-days-1-to-3.html