Possie Bugger (OFFB)

What is this pattern? It is normally tied in a size 8-10 or so. What could it be used for? It is highly thought of on special Oregon rivers. Caddis, Big Mayfly, Stonefly, Stillwater Dragon? The name must be an interesting story?

Posse Bugger by Jeff Kennedy

It must be a pretty good fly if worthy of a drawing by Jeff Kennedy