Thranduil é, na verdade, um nome inventado pelo próprio J. R. R. Tolkien.
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.
Obviamente, o nome é repetido na maioria das obras do Professor, mas sem dúvida o O Senhor dos Anéis é o primeiro livro publicado a publicá-lo, e talvez (não tenho certeza disso) ) é primeiramente concebido em O Silmarillion .
Como comentários de @Plutor , isso não prova que Thranduil é o mesmo Rei dos Elfos no Hobbit Podemos ler na citação abaixo como Glóin faz uma referência direta ao Rei dos Elfos logo após Legolas mencionar as masmorras onde elas eram mantidas por seu pai, confirmando assim que Thranduil é o rei élfico caracterizado no 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.