2016.02.04  “Elvenking” is used exclusively in Chapter 9 and onward; “Elf-king” is used only in Chapter 8.  This first reference may have been to a predecessor, the second is definitely Thranduil and “Elvenking” hereafter is exclusively Thranduil.

“Elf-king” is not attested in OED; “Elvenking” is, but in this manner, which gives all the credit to Tolkien:

2. Comb. (referring to a kind of imaginary being in the works of J. R. R. Tolkien).

a. appositive, as elven-kin.

b. attributive, as elven-king, elven-tongue; elven-wise adj.

1937   J. R. R. Tolkien Hobbit xvii. 285   But the Elvenking said: Long will I tarry.

• 08.133 for the elf-king had bargained with them to shape his raw gold
• 08.133 If the elf-king had a weakness it was for treasure,

“ˈelven, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/60661. Accessed 13 September 2017.