Yellowstone is chock full o' wildlife.
Buffalo are plentiful in the park and appear to be conditioned to people since they approach cars without hesitation. But there are many signs that warn about the dangers of getting gored by these wild beasts which can run up to 30 mph. Nuts.
While driving, we saw a group of mountain goats tearing up a rocky cliff-face at the roadside. They were bounding up and down this 60+ degree unstable slope as effortlessly as we walk up stairs.
Some more wildlife of note: a herd of elk, some deer, assorted small birds/fish, and a wolf tromping around a field, which Jay got a nice pic of.