The following is a list of probable inspirations for D&D monsters.
Displacer beast: Based on the coeurl from van Vogt's science-fiction tale, "The Black Destroyer," later incorporated into the novel, Voyage of the Space Beagle.
Orc: Orc is a term for an ogre (with which it is a cognate) or demon, from the Latin orcus. D&D's humanoid orcs were heavily influenced by Tolkien's Lord of the Rings, although they are distinct from them in many particulars. The orc's bestial, even porcine snout may have been influenced by popular LOTR covert from the mid 70s, or may be a play on the Welsh word for a big or boar (as in Orkney Islands, Island of the Boars).
Troll: Although trolls are a mainstay of Northern European mythology, the green-skinned, regenerating, bestial troll is drawn from Poul Anderson's The Broken Sword.
Zargon: This six-tentacled, clawed monster resembles the Lords of Luz from Lovecraft's Dreamlands.