The mythological creatures that plague a certain country or culture are said to have their basis in that society's fears. But some... some are just completely messed up.
In Japanese Buddhism, a greedy or selfish person becomes a jikiniki when they die. The name means “human-eating ghosts” but don't worry, they only eat dead humans. Someone call Ozzy 'cause this shit is metal.
Jikiniki persist by eating the flesh of the recently deceased as well as the offerings left out for the dead. However, they're not just cannibals, they're also thieves. Jikiniki will often loot corpses and use the valuables they find to pay off local law enforcement officials, because they've got an arrest quota when it comes to freaky ghosts.
They ghosts themselves look like cadavers (because subtly isn't big in the afterlife, apparently) with red eyes and sharp claws. Depending on who you ask, jikiniki also have the ability to mask themselves as regular non-rotting people during the day, allowing them to lead double lives. Despite this neat little magic trick, jikiniki are painfully aware of the monsters they've become and are incredibly self-loathing. Hosting remembrances can release a jikiniki from curse.
Lady Midday is the English name given to traditional demon throughout various parts of Europe, with roots in Slavic mythology. Serving as the personification of heat stroke, she would attack people working at noon by inflicting illness or madness, because where do you get off trying to make a living, asshole?
The lady appears in the fields as a whirling cloud of dust carrying a scythe or shears. When ready to strike, she will approach someone and begin asking them a series of difficult questions. Should she receive a wrong answer or the victim change the subject or ignore her, she'll simply cut their head off. On the upside, she would scare children away from crops, so there's that.
Depending on which part of Europe you hail from, she could appear in different forms: an old hag, a twelve year-old girl or a beautiful woman. In some parts of Europe she would also abduct children who would search for flowers among tall plants.
Coming from Indonesian mythology, lang suirs are the ghosts of women who died during or shortly after childbirth. In order to win this particular prize, both the mother and child must have passed. The would-be mothers then become the fabled ghosts forty days after dying, though this can be prevented by placing glass beads in the deceased's mouth. Only a choking hazard can thwart the dark powers of Satan, it seems.
The lang suir is incredibly ugly with red eyes and long claws. They attack pregnant woman, causing miscarriages or out-right murdering them in a two-for-one deal. What's more, they can suck blood through a hole in the back of their neck. Ignoring the incredibly odd posture that must require, that's pretty scary stuff. They appear by shores dressed in green or white and can take the form of an owl.
It's possible to turn a lang suir back into a human. The first method is to cut its claws. The second is a bit more difficult: take a strand of its own hair and jab it through the hole in its neck.
Tiyanaks are kind of like baby vampires gone straight to hell. Hailing from the Philippines, these creatures vary somewhat depending on local tradition, but all stories agree that the creature mimics the look and cries of a baby to lure people into the jungle. Then, when the tiyanak is held, it takes on its demonic form and flat out corpses you.
The “true” form of the tiyanak varies quite a bit depending on where you are. The most common is similar to an infant but with long claws and fangs. Another claims it can take on the form of a blackbird as well (though it can still fly as a baby, so go figure). The most complex description is that of a dwarfish creature with a flat nose and long beard, deformed with one leg longer than the other forcing it to leap after its victims. Regardless, the creature also makes a hobby out of child abduction. If you ever find yourself needing to defeat a gimped dwarf in the jungle, simply put your clothes on inside out. This apparently amuses the spirit enough to make it reconsider the whole “murder” thing.
Interestingly enough, the tiyanak's origin story has changed over time. Originally believed to be the spirit of a baby born to a dead mother (that is to say, the mother died before giving birth – ew), when Christianity was brought to the Philippines in the 16th century it was altered somewhat to be the spirit of aborted fetuses seeking revenge.
If there was ever a good reason to stay the hell out of Zanzibar, it's the Popobawa.
Popobawa is a shape-shifter that does a very good job of masking its presence. Practically undetectable until it's too late, it's said to enter homes through chimneys or keyholes and, once inside, proceeds to sodomize everyone inside, including children. Then, once finished with its mass violation, the creature warns its victims to tell others or it will come back and do it again. There's no nice way to say it: that's pretty fucked up right there.
Popobawa is only about forty years old and sightings of it are absurdly common, with panics happening every election cycle along East Africa. During these panics people will gather around fires outside of their homes and wait for daylight. Though it has no overt weakness, it refrains from attacking anyone who is holding a Koran. Some people will also sacrifice goats to the spirit.
The origins of the popobawa are a little murky, though the most common involves a sheik seeking revenge on his neighbors. He releases a genie only to have it accept demonic influences and start raping everyone. Say what you will about popobawa, but he cuts to the chase.
An interesting twist on mermaids, the rusalki are fish women from Slavic mythology. They dwell in the bottoms of rivers until nightfall when they would take to the land and dance in meadows. Should an attractive man travel too close to a dancing rusalka, she'll mesmerize him and lead him to his death, drowning him in her river.
Different regions tell the story with slight variation, but in general rusalki aren't intentionally malevolent and may not even be aware of their present state. Most traditions say they are the spirits of women who died before their time or those of unwed mothers, so in both instances they pine for the love of men as well as the company of little children.
Rasalki can also take the form of a child with the spirit itself resulting from child who was murdered because it was born out of wedlock or dying before it was baptized. Should you avenge the spirit's death (or baptize it if it's a baby) the spirit will return to rest.