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):
Cthulhu
Azathoth
Nyarlathotep
Yog-Sothoth
spawn of Yog-Sothoth
Primordial Ones
Shoggoths
Mi-Go
Great Race
Deep Ones
Unquiet Worms (kind of)
Derleth (total of 5):
Hastur
I'thaqua
Cthugah
Cthugah's flame creatures
B'yakhee
Clark Ashton Smith (total of 3):
Shub-Niggurath (Abhoth)
spawn of Shub-Niggurath (spawn of Abhoth)
Fetor of the Depths (Tsathoggua)
Robert E. Howard:
Snake-Men (extinct on Carcosa)
Gary Gygax:
Slime God (Juiblex)
F. W. Holiday's theory of the Loch Ness Monster:
Lake Monsters
My own (total of 27):
Lurker amidst the Obsidian Ruins
Deep Gibbering Madness
Putrescent Stench
It of the Fallen Pylons
Crawling God
Leprous Dweller Below
Shambler of the Endless Night
Inky Crawler
Lurker of the Putrescent Pits
Weird God
Violet Mist
God of the Primal Void
Tentacled One
Foul Putrescence
Suckered Abomination
Colorless Ooze
Watery Death
Desiccating Slime of the Silent Halls
Squamous Worm of the Pit
Amphibious Ones
Diseased Guardians
Green Ooze Pool
Mummies
Mummy Brains
Giant Jungle Ants
Space Aliens
Species 23750