In J.R.R. Tolkien’s legendarium, Shelob’s ancestor is Ungoliant.
Ungoliant is a monstrous, spider-like being who appears in The Silmarillion. She joins the rebellious Vala Melkor, later known as Morgoth, in attacking the Two Trees of Valinor. Ungoliant drains the light from the Trees and helps Morgoth destroy them before the events of the main narrative of The Lord of the Rings.
After the attack, Ungoliant demands the jewels Morgoth has stolen. Their alliance collapses when she tries to consume the Silmarils, and Morgoth escapes only after summoning Balrogs. Ungoliant then flees south and eventually becomes associated with giant spiders descended from her line.
Shelob, the giant spider encountered by Frodo and Sam in Mordor, is described as one of Ungoliant’s descendants. Ungoliant is not simply another name for Shelob; she is an earlier and far more ancient figure.