These are oversized spiders; the evil, intelligent children of the Spider Queen Shelob.
While they've taken Nan Ungol (Sindarin for "Valley of the Spiders") as their primary home, they've since spread across Mordor taking residence in its caves and other dark places, and are actively reporting back to their queen.
Their nests and lairs are littered with the crushed bones of their prey. They are not afraid to make victims of the encroaching Uruks.
The recent stirrings in Mordor have emboldened the Ungol and piqued the curiosity of the Spider Queen, but to what end remains a mystery.