It delights me even more, though, to hear that my nameless cosmic monsters have an air of originality about them! Shapeless, unheard-of creatures are not original with me; for although Poe did not use them, they figure quite widely in minor horror-writing since his time. Usually they tend to be exaggerations of certain known life-forms such as insects, poisonous plants, protozoa, & the like, although a few writers break away wholly from terrestrial analogy & depict things as abstractly cosmic as luminous protoplasmic globes. If I have gone beyond these, it is only subtly & atmospherically—in details, & in occasional imputations of geometrical, biological, & physico-chemical properties definitely outside the realm of matter as understood by us.
Source:Letter to Elizabeth Toldridge (March 8, 1929)