Shelob is the great spider, daughter of Ungoliant the primordial spider. Shelob's layer is in the mountains of darkness, in Mordor and she lived their long before the event of Lord of the Rings took place. When Sauron decided to set up his kingdom in Mordor, he new of Shelob's presence and let her stay there as an accidental guard to that pass.
Excellent climbing capabilities:
Stinger: has a stinger that paralyzes and knocks out its prey
Webs: spins incredibly