| 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, elletrć, 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. | 
