I went through the "Call of Cthulhu" bestiary last night and calculated approximate dice values for the Sanity risk each creature poses. I tried to alter the dice depending on each spread, but hopefully this will approximate well. The application is unclear at this time: presumably these would be monster dice used for Sanity conflicts (in "Hounds" terminology) or an outline for supernatural traits (in "Afraid") should the existing amounts be insufficient for Mythos use.
One thing I failed to do was list an example Mythos creature with each die type. "Save" means the original amount of Sanity lost on a successful save, "Fail" of course, means the amount lost on a failed save, "Max" was just the maximum loss potential, and "Hounds" is the die conversion for a "Dogs"-based system.
Save Fail Max Hounds
0 1 1 1d4
0 1 1 1d4
1 1d3 3 1d4
0 1d4 4 1d6
0 1d6 6 1d8 (common Mythos creatures)
1 1d6 6 2d4 (common Mythos creatures)
0 1d8 8 3d4
1 1d6+2 8 2d6
0 1d10 10 4d4
1 1d10 10 2d8
1d3 1d10 10 3d6
1d6 2d6 12 2d10
1d4 2d6+1 13 2d10
1d4 1d20 20 3d10
1d6 1d20 20 3d10
2 1d20 20 5d6
1d3 1d20 20 5d6
1d4 1d20 20 4d8
1d3 2d10 20 4d8
1 01-100 100 10d10 (Yog Sothoth)
1d10 01-100 100 10d10 (other Great Old Ones)