One of my personal goals for Hari Ragat was to stick strictly to pre-colonial material for the setting. Thanks to the works of scholars like Scott and Eugenio I was able to get into the epics pretty easily, but coming up with the bestiary was a bit more challenging.
A lot of Philippine lower mythology creatures acquired Spanish names and properties over time. Mermaids became sirena; dwarfs were duwende; evil spirits maligno; diwatas and wizards became encanto; ghosts were multo, from Spanish muerto; and so on. Popular Philippine culture has become so Westernized that Philippine dwarfs are often represented wearing Western-fairytale trappings, complete with pointy caps a la Doc, Grumpy, et. al.
Fortunately equivalent terms could still be found in different Philippine languages, specially the Lumad tongues, Tagalog and Bisaya. In some cases I went a little farther out, to the next closest culture which was Indonesian. Thus the replacement of sirena with duyung, which originally did mean mermaid. I guess the Spanish idea of the pretty Melusine-type mermaid was more appealing than the picture of the dugong, thus the switch!
Speaking of merfolk, the Philippine merfolk do seem more fearsome than the post-medieval Western archetypes. While the aspect of drowners and terrors of the sea became muted in Western merfolk stories as we remember them now, probably thanks to modern fairytale adaptations, Philippine merfolk definitely had a monstrous side. The siyokoy mermen were thought to be misshapen fish-men, more Lovecraftian Deep One types than Ariel’s pretty boys in the Disney movie. The twin-tailed mambubuno was a drowner of fishermen, its very name evidence of its brutal nature: mam- “one who,” buno, “wrestles.” The marindaga of Bicol was a mermaid with eel- or sea snake-like tail that lures fisherfolk to her then eats them.
Though I’m actually done with the main rulebook of Hari Ragat (yes, it’s done!), I’m always on the lookout for more mythic creatures for possible expansions. And I’m having a ball doing so.