Erl King is the English rendering of German
Erlkönig, made famous in the ballad by
Goethe set to music by
Schubert, but the German name was an error. It means
Alder King (
Erle). It was used by Goethe's friend, the philosopher, critic, and
folk song collector
Johann Gottfried Herder in his 1778 collection
Stimmen der Volker (Voices of the People). Herder translated the
Danish ballad of
the King of Elfland's Daughter, but mistranslated the Danish name
ellerkonge (also
ellekonge and
elverkonge) as if the first part was 'alder'. It actually meant '
elves'.
Looking up a Danish dictionary shows 'alder' = el, elletræ, and 'elf' = alf, so I suppose the el- form of 'elf' must be older Danish. This explanation of a mistranslation is given in many respected sources, so I don't doubt it's right.