The Penanggalan / Manananggal is a vampire-like creature found in Southeast Asia (the former in Malaysia, the latter in the Philippines, with some differences in behaviour). It is described as a flying woman’s head with a cluster of exposed entrails flowing behind her (which apparently twinkle like fireflies). Each night, this hideous freak attacks and drinks the blood of young children and pregnant women. Could something like this evolve in nature, especially the entrails (or at least gives the illusion of entrails)?
Forget for a moment that people think it is a vampire. Focus on this description alone:
a flying woman’s head with a cluster of exposed entrails flowing behind her (which apparently twinkle like fireflies)
If it flies with no wings, it may be a living balloon. If it shines, it has bioluminescence. All in all, I can picture a flying jellyfish. The closest thing ever in media are the Hanar people of the Mass Effect series of games.
Imagine if you will a jellyfish, about as tall as a human (counting the arms). Imagine that it is filled with gases, so that it may fly in air. This probably has to be handwaved.
This creature has evolved to prey on humans. It will stalk and hunt small children and pregnant women because they are more vulnerable - this hunter is kinda fragile and avoiding hard fights is a must for it. It will probably also hunt the elderly and those with special needs for the same reason.
So why a human face? Maybe the creature evolved human-like facial features so that it may arise less suspicion when peeking at people from far away, or behind windows or bushes. Or maybe, and this is the scary option, the creature is sensient and uses a human mask - or a human face, cut from a victim - for the same purpose.
As for the bioluminescence, the creature activates it while hunting to either see what it is doing (since it will initiate attack in the dark) or to scare and maybe stun other humans which might come to the rescue of a victim. If the legends say that the light can burn or kill you, it will make a hunt easier since people will be less likely to help a victim.
That this beast is a flying jellyfish actually makes sense in the vampire narrative. The monster's tentacles will not only inject poison, they will also inject digestive enzymes on a victim. The jellyfish will grab as much digested fluid from a victim as it can get and then fly away.
Also, since people believe that it is a vampire, they will try an array of superstitions to try and ward the beast off. I don't know if people in Asia think garlic and crosses will keep a penanggalan away, but whatever they do, it is probably innefective. Which is also to the benefit of the penanggalan.
The problem is that a simple head won't fly - it needs some wings. And that is exactly where those "entrails" come into play.
The "entrails" are really a form of wings that allows your creature to fly. Basically you will have some sort of very-large vampire bat, whose wings are strangely arranged and longer than those of present day vampire bats. These little critters then seek out young children and pregnant women to feast on their blood, because those are easy targets.
We are talking about a really big bat here - the body has to be big enough to be mistaken for a humanoid head. These not-so-little beasts are actively hunting for little/helpless humanoids.
As it's dark when they hunt people won't be able to easily see what they look like. Their fur is just coloured in such a way that people think it could be a human head - with enough fantasy and fear. The fact that they are feasting on blood and hunting for victims means that they might also get some blood on them in the process, which further leads people to think that these heads have bloody entrails behind them. The darkness helps to conceal that their wings are not completely loose chunks of flesh hanging behind them, just bigger, slightly deformed and strangely arranged.