There agelong she had dwelt, an evil thing in spider-form, even such as once of old had lived in the Land of the Elves in the West that is now under the Sea, such as Beren fought in the Mountains of Terror in Doriath, and so came to Lúthien upon the green sward amid the hemlocks in the moonlight long ago. How Shelob came there, flying from ruin, no tale tells, for out of the Dark Years few tales have come. But still she was there, who was before Sauron, and before the first stone of Barad-dûr; and she served none but herself, drinking the blood of Elves and Men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness. ... But none could rival her, Shelob the Great, last child of Ungoliant to trouble the unhappy world.
--The Two Towers, book IV, chapter 9: Shelob's Lair
A little way ahead and to his left he saw suddenly, issuing from a black hole of shadow under the cliff, the most loathly shape that he had ever beheld, horrible beyond the horror of an evil dream. Most like a spider she was, but huger than the the great hunting beasts, and more terrible than they because of the evil purpose in her remorseless eyes. Those same eyes that he had thought daunted and defeated, there they were lit with a fell light again, clustering in her out-thrust head. Great horns she had, and behind her short stalk-like neck was her huge swollen body, a vast bloated bag, swaying and sagging between her legs; its great bulk was black, blotched with livid marks, but the belly underneath was pale and luminous and gave forth a stench. Her legs were bent, with great knobbed joints high above her back, and hair that stuck out like steel spines, and at each leg's end there was a claw.
--The Two Towers, book IV, chapter 9: Shelob's Lair