Some works and artists that reference the "Erl-King":
Russian XIX-century poet Vassily Zhukovsky translated (or perhaps adapted, it wasn't a very literal translation) Goethe's ballad into a Russian-language poem "Лесной царь" ("The Forest Tsar") with the same plot.
The Dresden Files - A powerful member of The Fair Folk goes by this name and the poem is acknowledge to be about him in-universe.
John Connolly wrote a short story by this name about a boy who meets and just barely escapes the titular figure. Unfortunately, his infant brother doesn't have the same luck.
Sarah Brightman'' - Her song "Figlio Perduto" uses an Italian adaptation of this poem for lyrics.
Roommates - Also has the Erlkönig as character, and a meta webcomic adaptation of the poem, which is his official backstory. He would also love to hear about that Alternate Character Interpretation mentioned in the main tropes.
Doom Metal band Pagan Altar's "The Erl King" is an adaptation of the Goethe poem. Also, the Erl King is mentioned in "Armageddon" as being quite pleased with the nuclear war that destroys humanity.
Le Roi des aulnes: a 1970 novel written by Michel Tournier.
An episode in the fourth season of Boardwalk Empire is titled "Erlkönig", and parts of the poem are recited in it.
Of course pathos begets countless parodies (eased by the fact that the poem is so easily recognized even when parodied). To name just one, "König Erl" by famous German comedy poet Heinz Erhardt.
" by Epica from The Holographic Principle is based on the poem, with the spoken dialogue taken from there.