Thranduil är faktiskt ett namn uppfunnet av J. R. R. Tolkien själv.
There was also a strange Elf clad in green and brown, Legolas, a messenger from his father, Thranduil, the King of the Elves of Northern Mirkwood. And seated a little apart was a tall man with a fair and noble face, dark-haired and grey-eyed, proud and stern of glance.
The Fellowship of the Ring, Book II, Chapter 2: The Council of Elrond.
Now of old the name of that forest was Greenwood the Great, and its wide halls and aisles were the haunt of many beasts and of birds of bright song; and there was the realm of King Thranduil under the oak and the beech.
The Silmarillion, Of the Rings of Power and the Third Age.
Självklart är namnet upprepat i de flesta av professorns verk, men utan tvekan Ringenes Herre är den första publicerade boken för att få den på det, och kanske (jag är inte säker på det här ) det är först uppfattat i The Silmarillion .
Som @Plutor kommentarer, visar det inte att Thranduil är samma kung av älvorna i Hobbit. Vi kan läsa i citatet nedan hur Glóin gör en direkt hänvisning till kungen av älvorna strax efter att Legolas nämner fängelsehålen där de föddes av sin far, vilket bekräftar att Thranduil är Elven-kungen i Hobbit:
‘Not through lack of watchfulness,’ said Legolas; ‘but perhaps through over-kindliness. [...] But Gandalf bade us hope still for his cure, and we had not the heart to keep him ever in dungeons under the earth, where he would fall back into his old black thoughts.’
‘You were less tender to me,’ said Glóin with a flash of his eyes, as old memories were stirred of his imprisonment in the deep places of the Elven-king’s halls.
‘Now come!’ said Gandalf. ‘Pray, do not interrupt, my good Glóin. That was a regrettable misunderstanding, long set right. If all the grievances that stand between Elves and Dwarves are to be brought up here, we may as well abandon this Council.’
The Fellowship of the Ring, Book II, Chapter 2: The Council of Elrond.