Yellowstone carries a diversity of trout and char species, including several sub-species of cutthroats, brown trout, rainbow trout, brook trout, the invasive lake trout, and the artic grayling. Grayling appear in a few drainages and lakes in the area. My friends and I caught several of them at Cascade Lake, about two miles past Grebe Lake in the park. Unlike some late summer lake fishing, the grayling and cutthroats in the lake were cruising the shore line and eager to take dries. This one took a stimulator, but we got several on rusty spinners.
Grayling and cutthroats are similar to the eastern brook trout we find here in the Shenandoah. They are more than willing to take a dry fly if the presentation is somewhat accurate. Attractor patterns such as stimulators, triple-doubles, and adams are all that you need.