1 Buffalo
Because I grew up in Jackson Hole, seeing buffalo was a daily occurrence for me and it became as normal as seeing a cow. However, if you've never had the chance to see buffalo, you can appreciate how magnificent they really are. These creatures can run up to 35 miles per hour, and they can weigh up to 2,000 pounds. They do have unpredictable temperaments, which is why you always want to stay in your car when you see these animals in Yellowstone National Park. They are best appreciated from (safely) afar.
Although they can be aggressive at times, I think moose are some of the most stunning creatures in Yellowstone. They're large, weighing up to 1,000 pounds, and the males have big, imposing antlers. They also have very sweet faces. If you want to see moose on your trip, look in marshy areas with willows, and consider yourself lucky if you do see one, because there are only 200 in the park.
Seeing black bears occurs more frequently than you might think. Between 1910 and 1960, people would get out of their car and feed the bears, so there were a lot of black bears to be seen. Now, they're not so numerous, but they're still around and are quite adorable. I've seen black bears almost every time I've gone through the park. Again, stay safely away from these animals.
Bighorn sheep look like stocky deer with big, thick horns. You won't often see bighorn sheep in Yellowstone because there are only around 300 of them. But it does happen. When I was younger, I would see them occasionally standing tall and proud in the grass.
Elk are prolific in Yellowstone, and you're almost guaranteed to see at least one of them. However, if you want to be sure to see elk, you need to go to Jackson Hole in the winter where the elk graze on the National Elk Refuge.
Mountain goats are difficult to spot because they live in high elevations. They're furry and white with small horns. I've seen them in the fall on several occasions, climbing the sides of mountains, which is pretty amazing to see.
Wolves were reintroduced to Yellowstone from 1995 to 1997. They're hard to spot, but I was once lucky enough to see one. It was quite awe-inspiring because it was much larger than I expected.