The obvious answer is that yes, supernatural explanations are needed. But that answer is boring, so let's try something else.
The Manananggal is a species of bat that has evolved to disguise itself as human. Like a hermit crab and it's shell, they use an external pair of legs to keep up the disguise.
It evolved because this bat once discovered how to hide among prey, and live as a parasite amidst a human community. Done successfully, this proved a huge survival advantage, and so over the generations their disguises have gotten better and better. Their entire survival relies on appearing human, and the ones that can't keep up the act have been killed.
Physically, the Manananggal is a bat as large as a human's chest. They fold up their wings, they assume a human like position, and they can masquerade as the upper half of a human. Perhaps they use masks of human skin to aid their disguise, or they scavenge heavy hoods overwise. They have learnt to use human clothing and skin to hide themselves.
The bottom half is from the body of a dead person. The Manananggal digs up a human corpse to aid their disguise, and then uses their legs like a puppet. Hollow out the bones, stick your legs into there, you've got crude human legs.
It spilts because the bottom torso legs doesn't actually belong to the Manananggal. Instead, a Manananggal will just scavenge a deceased human and use it's body like a puppet, hiding in the upper half. Come night, and when it leaves its corpse shell, the legs are too heavy to carry. So it simply severs itself, reveals it's wings and flies off with the upper torso.
If ever its corpse becomes too decayed to be an effective disguise, the bat simply abandons it and find another. The actual Manananggal swaps from body to body as needed. They are capable burrowers, to dig up graves.
Naturally, since they're wearing human corpses, their appearance is that of an extremely disheveled old man/woman. They appear unkempt, rough and smelly, but human. They pretend to be frail, infirm and near incapable of moving, such that they are nursed by a community. They know that humans are social creatures that will care for the elderly, and so the bat exploits this.
This bat is a parasite that hides in a community, yet every night they reveal their true form and drain a person's blood for food. Naturally, the Manananggal targets the vulnerable - the young, the old and the pregnant.
It uses its wings to assault sleeping prey at night, and then return to its old woman ruse on a morning.
Provided nobody discovers it, it's an effective disguise. The human community provides protection and shelter, as well as a convenient food source. The bat simply has to pretend to be a frail old lady with trouble walking.