The Whitlock Red Fox Squirrel nymph is listed as a standard in a lot of places, including Dave Hughes' Trout Flies. But the material doesn't seem to be that easy to obtain. I've never seen it locally, and an internet search only turned up a few places -- none that I've traded with before.
On the other hand, the colors in the photographs seem to approximate an GRHE nymph.
I claim that you could tie a Fox Squirrel nymph with hare's mask dubbing, and that there's no reason to carry both flies.
Am I right about that?