|"Nurarihyon", by Sawaki Suushi - 1737|
But that still doesn't answer where the elongated cranium image came from. So, was there a real person with a birth defect who became known as a yokai later, the way many battles won by many different men in time become 'known' as the ventures of one single man in history (ala Emperor Jinmu?) I had to find out.
Turns out, artificial cranial elongation (ie, distended the skull as seen in the painting) is a real thing! Yep, it really happened. Throughout various cultures and time frames, the practice of lengthening the skull was seen again and again... including in one region of Pacific islands, Vanuatu and Malakulan, off the coast of Australia. Is it unreasonable to think that perhaps, a long time ago, someone who had their skull lengthened was swept away on their boat during a storm and wound up near Japan's shores? Being a socially-isolated country for so long, perhaps some Japanese fisherman came across such a person, and believing in the supernatural, a new yokai came about.
How is cranial elongation done? Well, you see, when a person is born, the skull is very soft and hasn't fused into place yet. This is why infants die so easily from what to adults would be relatively minor injuries to the head. Starting in infancy, some cultures adapted ways to change the shape of the skull, either through binding or pressing it for months at a time to get it to grow a certain way until later towards adulthood, when it would begin to fuse together permanently. In Malakulan, it is believed that a person who has this done is closer to the spirits and is more intelligent than others in the tribe.
There are some pretty cool photos of actual skulls at the Wikipedia page, and plenty of photos of living people who have had this done. It's a fascinating practice, although, maybe not one I'd participate in.