On my quest to make pretty much every single fantasy creature biologically feasible I have come across a hiccup. It wasn't angels, no you people helped me with that. Demons are easy, we have plenty of ways to fireproof a human. No it was something much less common, something people may know, of but don't know the name of. The Dullahan, also known as, The Headless Horseman.
Originating in Irish mythology, which is a field day on its own I'll tell you that, most of his supernatural features are pretty simple to explain. for example:
Quotes Cited from: https://en.wikipedia.org/wiki/Dullahan
However this does not lend itself heavily to the fantasy depictions seen today.
uses a human spine as a whip
Now that is the easiest part to explain, human spines are not that hard to come across if you look hard enough.
Drags a wagon adorned with funeral objects
There are plenty of funeral care stores around the country, so this is easy enough.
All of these features are cosmetic however. It is all "things made of dead people" that is simple. But if we get down to the meat and potatoes of a Dullahan. If we look deeply there are only two to three elements that bare any weight. The head, the body and the horse. And we can do without the horse.
Say there was a race of these Dullahans. Say they are no longer grosser Grim Reapers, but instead just a person or other biological sentient being that could live with a head that at least seemed to be able to detach. A Dullahan could be seen and reported to have their head detach and be held in their bodies hands.
Now some mythology has the Dullahan have no head at all. That is simple: move the brain and sensory organs to the body and Bob's your uncle. But I am looking for something challenging here. there are two depictions of the Dulahan, one where they have control of both the head and body and the second is a depiction in which the head has control of the body while attached but once removed the body has a mind of its own. Most commonly seen as less intelligent than the head, these bodies are always seen responding to orders yelled at the body to put the head back on top, with the success of these orders being dubious at best.
My question is, how would this be possible? How could a creature be capable of living and at least having basic problem solving and motor skills without a head. The furthest I could get harkens back to the masterpiece of a film, Pacific Rim, in which the theory is proposed that large creatures such as Dinosaurs and the Kaiju required a second brain to handle motor control in such an unwieldy body. This is as far as I have gotten so maybe the smart people over at Stack Exchange can give us a little help.