What follows are McKinney's words, not my own.
1. You wrote: "most Carcosan monsters have psionics"
Out of the 48 monsters detailed in the book, 15 of them possess psionics. Since only 1 in 100 of the random Spawn of Shub-Niggurath have psionics, I did not count them amongst the 15.
2. Regarding the monsters in CARCOSA you wrote: "most of them drawn from the writings of H.P. Lovecraft, along with a handful of others unique to this book."
Here is a break-down of the origins of the monsters in Carcosa:
Lovecraft (total of 11):
spawn of Yog-Sothoth
Unquiet Worms (kind of)
Derleth (total of 5):
Cthugah's flame creatures
Clark Ashton Smith (total of 3):
spawn of Shub-Niggurath (spawn of Abhoth)
Fetor of the Depths (Tsathoggua)
Robert E. Howard:
Snake-Men (extinct on Carcosa)
Slime God (Juiblex)
F. W. Holiday's theory of the Loch Ness Monster:
My own (total of 27):
Lurker amidst the Obsidian Ruins
Deep Gibbering Madness
It of the Fallen Pylons
Leprous Dweller Below
Shambler of the Endless Night
Lurker of the Putrescent Pits
God of the Primal Void
Desiccating Slime of the Silent Halls
Squamous Worm of the Pit
Green Ooze Pool
Giant Jungle Ants