Erl-king (n.)

1797, in Scott's translation of Goethe, from German Erl-könig, fiend who haunts the depths of forests in German and Scandinavian poetic mythology, literally "alder-king;" according to OED, Herder's erroneous translation of Danish ellerkonge "king of the elves." Compare German Eller, Erle "alder" (see alder).