Hit an amphibian grand slam-- although some are less commonly seen than others, the grand total is a whopping four:

Boreal toads, Bufo boreas boreas
Columbia spotted frogs, Rana luteiventris
Blotched tiger salamanders, Ambystoma tigrinum melanostictum
Boreal chorus frogs, Pseudacris triseriata maculata

And a very brief pause and pass through Utah, produced a couple more:

Great Basin gopher snake, Pituophis catenifer deserticola (recent DOR)
Tiger salamander, Ambystoma tigrinum nebulosum

I am a tiger salamander enthusiast and wanted to see tigers more than any of the other amphib species. This backcountry pond and smaller connecting ponds, adjacent to Yellowstone Lake, was a very productive breeding site for all amphib species:

Blotched tiger salamander larvae and spotted frog tadpole

Cutthroat populations are down significantly due to introduced lake trout, and probably drought and other factors to a lesser degree—but they were still there and were on average bigger than usual (only a good thing in the short run). On the river, many were still donning their bright spawning colors.

Of course, all the other denizens of the Park were also there—moose and deer waltzed through our backcountry campsite daily. Some of the family saw a grizzly, but I was fishing at the time and missed it.

Every night, boreal toads, which were not found during past surveys in the area, could be found around where we made camp—sometimes hopping along the gravelly shorelines of Yellowstone Lake. I didn’t get any good pictures unfortunately.

(Rest off-topic) The last day, we happened onto a great brown drake (mayfly) hatch on the Yellowstone River, which made for incredible dry fly fishing.

Two herds of buffalo passed behind us on the bank as we fished, one bull took a few paces into the river at us… I hardly blinked and I was back in the office, 1000 miles away.

Also while some of the Park literature inaccurately says there are no Rattlesnakes in Yellowstone--Wayne and I have found them inside the Park.

I'm not aware of any "reliable" records of Coluber constrictor in the Park. I can think of areas that would suit them, however.

It is true that 99% of Park visitors however probably will not traverse the area of the park where rattlesnakes can be found (unless perhaps they cross marginal rattlesnake range in a 1/2 mile stretch of the road near Gardiner).

In the very small area of the park where rattlesnakes are present, they are far from rare and can be found in concentration.