Beethoven composed several songs on poems by Goethe and for uncertain reasons a few of them remained unfinished. He worked on Erlkönig when he was 22 years old and probably laid it aside because he had some commercial works to do first. He never got back to it.
I reconstructed this song back in 2002 and it has been performed in public a few times since then, but it was never issued in print..until now! It can be ordered here.