50039: Gug, Eldritch Horror (can't believe you guys missed this one in your list)
02983: Familiar Pack IX (6) has a little Cthulhu in it!
02838: Faceless Horror also works for a flying polyp (grounded, of course)
02919: Burrowing Horror with a sculpted beard of tentacles would make an excellent Bokrug
02252: Wind Elemental with a couple of intense eyes could pass for Ithaqua; alternatively, you could flatten the chest and remove the tombstone of either 14148: Railor, Necropolis Sergeant or 20017: Ghost
03478: Tawny Firehair, Cat Girl as a priestess of Bast?
03425: Spell Effect: Burning Sphere as Cthugha? or the living pillar of flame?
03041: Mushroom King, an ally of the Mi-Go from Yuggoth? together with 02679: Mushroom Men and 02695: Fungus Patch
14188: Ssudai, Nagendra Mage = serpentman cultist
Other cultist candidates that seem appropriate for the age (there are a couple of others):
02797: Taenar, Cultist Leader
03312: Townsfolk: Cultists and Victim
03438: Darkspawn Cultist and Minion
03470: Khalatine, Evil Cultist
06175: Darkthrall Cultists (9)
14319: Darkthrall Cultist, Mercenaries Grunt
02718 or 65049: Garravank, Ghoul King would make a cool Y'Golonac if you hacked his head off (but it's such a cool head!) There's another fig, too: 03388: Garravank the Ghoul King
03049: Huge Spider for a Leng Spider? (check out the other spiders, too) Or... 03180: Cadirith, Colossal Demonic Spider
02133: Water Demon as Mother Hydra (maybe in ceremonial mask?) She looks a lot like the 02219: Domnu Of The Slithe fig which could be Dagon...
03086: Nephal Shadow Prince Demon without the sword could be Hypnos
One of the thousand faces of Nyarlathotep?:
02206: Pazuzu
02684: Frog Demon as Tsuthoggua
02868: Eldritch Demon is clearly Spawn of Cthulhu
You might be able to mold a Mi-go easily enough from 02744: Cichastus, Fly Demon. Replace the head with a bare brain and give it some crab claws. There may need to be another pair of legs, I'm not sure.
Wendigo:
02697: Yeti
03029: Sasquatch
14574: Yeti Shaman, Icstd
50011: Sasquatch
Beetles for New Age Yithians (the brood from the future):
02492: Scarab Beetle Swarm
02564: Giant Scarab Beetle
02844: Acid Beetles (2)
03256: Dungeon Vermin
03262: Beetle Swarm and Victim
Add greenstuff to cover the face over of a bat-winged gargoyle or demon and you've got a nightgaunt: 02039: Gargoyle or 03223: Gargoyle or 20031: Gargoyle
And these are just too cool to miss:
02837: Haunt
14547: Night Spectre
03230: Goroloth
02943: Festering Spirit
60007: Sinspawn