Just out of interest, here is how Tolkien describes Shelob's origins and nature:
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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 Luthien 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 there before Sauron, and before the first stone of Barad-dur; 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. Far and wide her lesser broods, bastards of the miserable mates, her own offspring, that she slew, spread from glen to glen, from the Ephel Duath to the eastern hills, to Dol Guldur and the fastnesses of Mirkwood. But none could rival her, Shelob the Great, last child of Ungoliant to trouble the unhappy world.
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This does suggest that Shelob is more than a merely large spider. She is the last child of Ungoliant - possibly something of a Half-Ungoliant, the last of the spider Peredhil, as t'were. She seems to have exceptional long life, maybe even to have endless life (and does she die after this? I think maybe not). She both eats traditional prey and vomits darkness. She has no master and follows nobody but herself; like Ungoliant she is something of a mystery.
As an aside, this is a quite beautifully written passage. Beginning with a rich aura of legend, mentioning Beren and Luthien, it builds slowly into the horror that Shelob presented to the unwary.