I found this blog looking for pictures of Jungle Jim. Now, there are some truly strange Jungle Jim adventures by Wally Wood and company, and some other stuff he did, but down a ways there are some pages from the Prince Valiant newspaper strip. If you look at the one from December 25, 1937, you will see a familiar face...
http://potrzebie.blogspot.com/search/label/wood%20chips
Wow. That's weird! Wonder if that was the inspiration?
Apparently, Kirby always said it was. So Etrigan has goosefoot ears...
Really? I had no idea Kirby had given that strip credit for Etrigan's look! That is too cool. :D
I saw it and was surprised, but if you google "Prince Valiant Jack Kirby Demon", you find quite a lot about it.
In the late eighties*, a bit after discovering the Demon through Moore, Bissette and Totleben's Swamp Thing, I picked some old French translation of Prince Valiant which included this page. I certainly had to do a double take on that one! :D
*Yes, Daglob isn't the only one who was already born at that time :P