This is a series of articles that focuses on mythical creatures of the Philippines that can be used in your RPG games or fantasy/science fiction/horror stories.
The fey have been known to abduct newborn babes from their cradle and replace them with Changelings, taking on the shape of the babe and causing all sorts of mischief in the family. In the Philippines, there is a similar phenomenon but these shapeshifters are more feral. Cute and cuddly by day, Tiyanaks transform into wild and feral beasts (think babies with sharp teeth and claws) when their prey is most vulnerable, such as when the mother is breastfeeding the creature. The tiyanak is also capable of mimicking the voice of babies and thus lead unwary travelers to its location in a forest, making them think that they are coming to the rescue of a defenseless baby. Aside from that, they are for the most part glorified gremlins.
The Tiyanak's strength is the fact that it can disguise itself as a human baby. A creative GM or writer can easily weave a scenario where a family has been mysteriously massacred (no signs of forced entry, the household isn't reaching for their weapons) and the only survivor is a seemingly innocent babe. The situation might take a turn for the worse if the heroes turn over the baby to some friends or acquaintances for safekeeping, unwittingly sending them to their doom. The Tiyanak isn't particularly invulnerable and is very much mortal but discovering its true nature is the real dilemma. The Tiyanak tends to strike at night or wherever the darkness can conceal its features when it reverts to its feral form. If you don't want to delve too much into Philippine myth, the Tiyanak can easily be a variant of the Changeling that showcases the true horrors of the Unseelie Court. In science fiction, Tiyanaks could be children that were infected by a rare virus strain that mutates them into carnivorous creatures if certain conditions occur.
Variations: In local myth, the Tiyanak slays women by biting and feeding on their nipples when they are breast-fed. It then escapes by transforming into a bird afterwards. A less malevolent version of the Tiyanak is that it appears in the guise of a child, typically in the forest, and leads travelers astray.