Did Tolkien give Thranduil his name?

Question

I am aware that during the writing of The Hobbit, Tolkien must not have had much in mind for a large universe, as he was simply writing a children's book. Therefore, Thranduil was simply named 'the Elven-king'.

I'm not entirely sure whether or not he was mentioned as Thranduil in any of the LotR books (I have not read them), but I'm pretty sure he was mentioned by that name in the extended first film. If so, this leads me to believe that either Jackson created the name himself, or that the name was created before that (either by Tolkien or someone else), just not when the character was introduced.

So, my question is, did Tolkien give Thranduil his name? If not, who did?

Answer 1

Thranduil is, indeed, a name invented by J. R. R. Tolkien himself.

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.}

Obviously the name is repeated in most of the Professor's works, but arguably The Lord of the Rings is the first published book to have it on it, and maybe (I'm not sure of this) it is firstly conceived in The Silmarillion.

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As @Plutor comments, this doesn't prove that Thranduil is the same King of the Elves in the Hobbit. We can read in the quote below how Glóin makes a direct reference to the King of the Elves right after Legolas mentions the dungeons where they were kept by his father, thus confirming that Thranduil is the Elven-king featured in the 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.}

Answer 2

Tolkien came up with the name Thranduil, but it was a rather late change, likely only created while writing the Appendices of The Lord of the Rings and then back-added into the text of The Fellowship of the Ring shortly before publication.

As published, "Thranduil" is mentioned four times in the main text of The Lord of the Rings.

1. 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.
   _{The Lord of the Rings - Book II Chapter 2 - "The Council of Elrond"}

2. 'Escaped?' cried Aragorn. 'That is ill news indeed. We shall all rue it bitterly, I fear. How came the folk of Thranduil to fail in their trust?'
   _{The Lord of the Rings - Book II Chapter 2 - "The Council of Elrond"}

3. Elrond is sending Elves, and they will get in touch with the Rangers, and maybe with Thranduil's folk in Mirkwood.
   _{The Lord of the Rings - Book II Chapter 3 - "The Ring Goes South"}

4. 'Welcome son of Thranduil! Too seldom do my kindred journey hither from the North.'
   _{The Lord of the Rings - Book II Chapter 7 - "The Mirror of Galadriel"}

However, in the original drafts, none of these had the name Thranduil, and were only added in after Tolkien finished the whole book, and began working on the fair copy and typescript.

1. Line does not exist in any of the five drafts of the chapter.

2. 'What!' cried Aragorn in angry surprise. 'Then all my pains are brought to nothing! I judge that to be evil news indeed. You may mark my words: we shall all rue this bitterly. How came the Wood-elves to fail in their trust?'
   _{The Treason of Isengard - Chapter VII - "The Council of Elrond"}

3. Elrond is sending Elves, and they will get in touch with the Rangers, and maybe with the Elves in Mirkwood.
   _{The Treason of Isengard - Chapter XIII - "Galadriel"}

4. Line does not exist in any of the two drafts of this chapter.

In The History of the Hobbit, John Rateliff suggests that the creation of the new name Thranduil may have been done at the same time that Tolkien had first consciously decided that Elvenking and Thingol were seperate people, something that he says there was no definitive evidence of before this point.

And some years later, Tolkien wrote up an in-universe etymology for the name Thranduil, saying that it translated to "vigorous spring".

S th(a)randuil 'vigorous spring', tharan 'vigorous'
_{Parma Eldalamberon #17, page 27}

√THAR-, vigour, only in Sindarin. tharan, vigorous, theria-, be vigorous, flourish. Hence th(a)randuil 'vigorous spring', a name.
_{Parma Eldalamberon #17, page 187}