- Elves as Shoggoths: They were created long ago by the Elder Things deep under the ocean. They rebelled and rose up to make a civilization of their own. Due to horizontal gene transfer (I got this idea off a Lovecraftian science blog post), they are the progenitors of most life on the planet and many of the intelligent species in the world teleologically converge into a humanoid form like their own. Their original shoggoth biology also makes them easily adaptable to different environments (explanation for why there are so many variant kinds of elves). Due to difficulties with reproduction (and possibly other factors) they are less prevalent than they once were.
- Dragons as Shantaks: Large, bird-like flying reptiles from the dreamlands. May be gryphons instead of dragons.
- Demons and ghosts as Nightgaunts
- Dwarves or halflings as Voormi: Small shaggy humanoids who worship a lazy, slimy, frog-like god Tsathoggua.
- Vampires as humanoids infected by Yog-Sothoth: It is the gate, interfacing with physical reality like a virus. Those infected attempt to drain life and energy in order to open the gate.
Admittedly some of these I've given varying degrees of prior thought to, or have been explroed before by others. Anyway, I've already got some ideas for a more developed version, but I think this isn't a bad start.