From Tolkien Gateway
The main article for this subject is located at Théoden.

Thengling is a name used in the Song of the Mounds of Mundburg poems, used in reference to Théoden's death at the Battle of the Pelennor Fields. It identifies Théoden by his father Thengel's name with the diminuative ending 'ling' to denote him being Thengel's child.[1]