Crossposted from a Yog-Sothoth.com forum post.
Human-alien hybrids in Call of Cthulhu... alphabetically.
ABHOTH, SPAWN OF: A human infected with the Abhoth-like degradation of the flesh, rendering it like rubber or tripe. This probably extends to nerves and brain-tissue, creating odd absences of human feeling or cognition.
ADUMBRALI: 'Seeker,' already covered.
AIHAIS: Possibly deriving from the same ultimate root, if mi-go experiments were responsible for the existence of humanoids on Mars. Human-Aihai hybrids might have functional second-limbs in the middle of their torso (or emerging from their second torso, depending on Aihai anatomy) and might be taller than the norm, although their height might be a response to the low gravity on Mars, in which case it depends on which planet they grow up on.
ASKALI: Might require artificial means of hybridization, but if that's done it the Askali single-eye might emerge out of the forehead, or one of the hybrid's human eyes might swell asymmetrically to accomodate it. (If the eyes *aren't* weird, you can't really tell it's an Askali, can you?) Might be related to Lemurians.
ANIMICULI: Already covered.
ATLACH-NACHA, DAUGHTERS OF: Already covered.
BYAKHEE: We don't really know where they come from; maybe they already are human-alien hybrids? They certainly resemble the human pattern more than you would expect a real alien to. Perhaps this is ultimately another mi-go tool built from human biomass, freed en masse by the Brotherhood of the Yellow Sign for their own interests, or some alternate form of Unspeakable Possessor originating in the unknown biome of Yhtill.
CHAKOTA: Already covered, but there's some Chakota-hybrid-like (mini-Chakota?) monster in TUO 14/15 (IIRC)
CHAUGNAR FAUGN, BROTHERS OF: Isn't there already a form of Chaugnar-hybrid? Or is that a unique entity?
CHILDREN OF THE WIND: These sound like they might be a form of Ithaquan servitor, not unlike the (false?) wendigo. Maybe this is what happens when they're stuck in a place (Syria) that's cut off from the influence of Ithaqua?
CHTHONIAN: Leto II.
COLD ONES: Might as well be the undying remains of the human cultists of the Pole-Flame.
COLOURS OUT OF SPACE: I think this is covered in one of the Secrets books?
CRAWLING ONES: Already covered.
CRYSTALIZERS OF DREAMS: Shadow people, natch.
CTHULHU, STAR-SPAWN OF: See CTHULHU, THRALLS OF.
CTHULHU, THRALLS OF: Already covered.
DARK ONES: Already covered.
DARK SARGASSUM: See Boats of the Glen Carrig.
DEEP ONES: Already covered.
DESH: Not unlike a ghost, the human-desh is only partially in and partially out of material reality. Parts of their flesh may sublime into Tillinghast-space or rotate through dimensions (it takes a POW roll to do so consciously, as the human-desh is an awkward hypershape). Unfortunately, human-desh are natural conduits for regular desh, and though their deshness makes the process nonfatal, it is debilitatingly painful and distorts the human-material sections of their bodies in the process.
DHOLES: Dhole-like characteristics emerge out of a human after infection by dhole genetic material. If the body adapts to the replacement of carbon by the silicon-like formation dholes are comprised of, they begin to grow thick pads of skin and losing sensation throughout their bodies. While this may seem advantages in the short term, the human-dhole eventually transforms into a thick knot of dhole-skin, immobilized by their own bodies.
DIMENSIONAL SHAMBLERS: As Desh, although the humanoid shape of a Shambler makes me think that there's some human somewhere in there already.
DUST MEN: Seem pretty similar to the Paramentals of Lieber, right?
DWELLERS IN THE DEPTHS: See CTHULHU, THRALL OF.
EIHORT, BROODLINGS OF: Already covered.
ELDER THINGS: An Elder Thing seed that somehow takes root in a human may, by virtue of the extreme antiquity of its biology, displace human organs with their own functioning, and eveventually hijack the genetic expression of new cells, growing elder-thing eyestalks out of human eye sockets and healing cuts with thick, fibrous knots of cactuslike flesh. EDIT: And of course, the Elder Thing seed doesn't trigger any immune-system responses due to admin-user privileges buried deep in the shoggoth-code.
FIRE VAMPIRES: These beings appear to be normal humans, but they have a complex electro-Kirlian field extending outwards. Electronic devices short out near them, and they can use the field to increase temperatures around them to dangerous levels—or suck the electrical energy right out of you.
FISHERS FROM OUTSIDE: Slenderman.