second attempt at a lasting response:

I asked about 50 students what comes to their minds when they hear "Erlkönig". Nearly all of them gave the response "the poem", none of them wanted to define what an Erlkönig might be. Few additonal responses were "a car" (also correct: "Erlkönig" is used in recent years for cars tested on the streets before they can be bought; the link is: they are only tested in fog and night when nobody can spot ot photograph them).

"Erle" is German for alder tree (that was my association as a boy and I pictured him as a wobbling ghost behind foggy alder trees), but I have no idea whether there is a connection.

Wolfgang