Aswang is an umbrella term for various shape-shifting evil creatures in Filipino folklore, such as vampires, ghouls, witches, viscera suckers, and transforming human-beast hybrids (usually dogs, cats, pigs). The aswang is the subject of a wide variety of myths, stories, arts, and films, as it is well known throughout the Philippines. Spanish colonists noted that the aswang was the most feared among the mythical creatures of the Philippines, even in the 16th century. Although with no specific motive other than harming others, their behavior can be interpreted as an inversion of the traditional Filipino's values. The aswang is especially popular in southern parts of Luzon, and some parts of Mindanao and Visayas, especially the Visayan province of Capiz.

"The sixth was called silagan, whose office it was, if they saw anyone clothed in white, to tear out his liver and eat it, thus causing his death. This, like the preceding, was in the island of Catanduanes. Let no one, moreover, consider this a fable; because, in Calavan, they tore out in this way through the anus all the intestines of a Spanish notary, who was buried in Calilaya by father Fray Juan de Mérida.

The seventh was called manananggal, and his purpose was to show himself at night to many persons, without his head or entrails. In such wise the devil walked about and carried, or pretended to carry, his head to different places; and, in the morning, returned it to his body remaining, as before, alive. This seems to be a fable, although the natives affirm that they have seen it, because the devil probably caused them so to believe. This occurred in Catanduanes.

The eighth they called osuang, which is equivalent to 'sorcerer;' they say that they have seen him fly, and that he murdered men and ate their flesh. This was among the Visayas Islands; among the Tagalos these did not exist."

Fr. Juan de Plasencia, Customs of the Tagalogs (1589)

Asuang. It is the name of an elf, sorcerer or demon, known in almost the entire archipelago, particularly among the Tagalogs, Pampangos, Bicoles, Visayas and Mandayas. It is a nocturnal demon that takes the forms that it wants, like those of a dog, cat, bird or other animal. The Asuang preferably picks up abandoned children and lonely walkers. With his tongue, horribly dilated, black and flexible as silk, he removes fetuses from women who are on tape. The birth pains are attributed to him. Many very different and even contrary fables refer to the Asuang, because the civilized nations of the Archipelago confuse under the name of Asuang the memories of various demons other than their ancient and primitive religion; so that today Asuang is usually, for both Spaniards and Indians, the generic denomination of a series of goblins, or equivalent to the meaning of the Spanish voice sorcerer or goblin. It seems that in Tagalog it was Osuang or Usuang, the oldest name that that Malaysian demon also carries in the Dutch countries of Indonesia. The bird Tiktik, night bird, announces with its song the proximity of the Asuang.

Ferdinand Blumentritt 1895

According to Maximo Ramos, the term "aswang" can be thought of as an aggregate term for a multitude of Filipino supernatural creatures. These creatures can be organized into five categories that parallel creatures from Western traditions. These categories are the vampire, the self-segmenting viscera sucker, the weredog, the witch, and the ghoul.