I've never caught many grayling in the Horse, but the House is great as is the hangingstone - I fish them where they cross the Highway south of town, i rarely fish for them at the mouths of those rivers except in the Fall they hit pretty good then.
I also fish for them up the Clearwater,, but you need a boat for that.
Anything bushy like a Tom Thumb, Elkhair Caddis, etc works pretty good.
A buddy of mine swore by a white dry fly "drowned" fished wet however.
Cat