This is a wet fly that I tie for Sea Run Cutthroats, as well as, West Slope Cutthroat Trout.
(I've also, taken a few Rainbows with it, but not regularly).
It is a known pattern, by a known fly tier.

The tail and body materials are quite obvious as to what they are, but can you guess the white hackle material?
Hint: It's very readily available, but fly shops don't carry it. (Neither do, craft, sewing, or, hobby stores).

(You commercial and/or, older fly tiers, may guess this one right away!?)