"Erlkönig" (also called "Der Erlkönig") is a poem by Johann Wolfgang von Goethe.
By wiki I know that the word
könig means 'king', but what's the meaning of
erl? Is it devil, elf, or genie?
The ballad Erlkönig is originally Danish and named there "Ellerkonge"... meaning elf-king. Eller was falsely translated with "Erle" which is a German tree. So the short form of "Erle" is "Erl".
Hope that could answer it :)