The great spider Frodo encounters in J.R.R. Tolkien’s The Lord of the Rings is Shelob.
Shelob lives in a tunnel above the pass of Cirith Ungol, on the route toward Mordor. Gollum leads Frodo and Sam into her lair, intending that she will kill them so he can recover the One Ring. Shelob attacks Frodo, but Sam uses Sting and the Phial of Galadriel to drive her away.
Tolkien describes Shelob as an ancient descendant of Ungoliant, the monstrous spider-like being who appears in The Silmarillion. Shelob is not simply an ordinary animal enlarged by magic; she belongs to a supernatural lineage connected with darkness and the destruction of light.
Ungoliant is therefore a related but different character. Mirkwood is a forest, not a spider, and Scatha is a dragon from Tolkien’s wider legendarium. Shelob is the specific creature in Frodo’s encounter.