The Erkling is a small elf-like creature native to Germany. Its high-pitched laugh is particularly entrancing to children, which Erklings like to eat. Erkling killings have decreased dramatically over the last few centuries as the German Ministry of Magic has put in place strict controls over the creatures (FB).
Rowling has transposed a few letters in the name of the Erl King ... of German legend. Otherwise, her description holds true. It is an evil creature in the Black Forest of Germany that tries to snatch children. (Magical Worlds of Harry Potter, p. 26).
The suffix "-ling" is often used to indicate that something is small, miniature, or otherwise diminutive.
Notions of elf- or fairy-like creatures that lure children away from their parents or guardians and sometimes eat said children appear in folklore worldwide. Notably, the Erl-King is a character from German Romantic literature of the 18th century. The Erl-King, or King of the Elves, lures children to their death.
From the Web
Erlking from Wikipedia