There are over 600 mayfly nymph species, of which most are brown, and very similar in appearance to the Pheasant Tail Nymph.
The CDC Flashback Pheasant Tail Tungsten Bead Nymph is an improved version that includes a copper tungsten bead, used to sink the fly quickly, and CDC soft hackle to add movement as the fly drifts in the current.
Use this pattern as a mayfly nymph imitation or attractor nymph, and plan on getting bit!