The name of the mythological creature in my novel is "manananggal". When I'm referring to it, should I capitalize what it is?
Mythological creatures, or creatures you've invented, don't need to be capitalised, just like real-life animals. There's no grammatical difference between "a dog", "an orc", "a dragon" and "a manananggal".
To cite a famous example,
In a hole in the ground there lived a hobbit. (J.R.R Tolkien, The Hobbit, chapter 1 - An Unexpected Party)
Do not capitalize the name of a species
There are hobbits, dwarves, dragons, horses, dogs, unicorns, cats, and so on.
Capitalize the personal name of an individual
They saw Peter, the human being.
They saw Capper, the dog.
They saw Smaug, the dragon.
Smaug is the personal name of this dragon. Other dragons have other names.
Note that while there may be more than one Peter (and potentially more than one dragon named Smaug), there is no species of peters (or smaugs). The same name can be given to different individuals (of different species: Peter the dog, Peter the dragon), but that still doesn't make them members of their own species, nor Peter a class name. They are all Peters, but not peters.
Capitalize the species names of unique beings
Grinch is not the personal name of this creature, it is the name of its species. But there is (or appears to be) only one of its kind, which makes its species name like a personal name in that it denotes a single unique individual. When I say "dragon" you don't know which one I speak of, but when I say "Grinch" you know which one I mean because there is only one of them, which makes "Grinch" function like a personal name.
Now you could say that the Grinch is a fictional character, invented by a writer of literature, and that its author may have chosen to capitalize this word on a whim. But there are examples from the real world that show the same difference in capitalization between the name of a mythological species and the name of a unique mythological being, such as banshee and Cailleach. There are many banshees. It is the name of a species and is therefore not capitalized. But there is only one Cailleach. Yet Cailleach is not the personal name of this being, but a description: cailleach means "old woman" in Irish, just as banshee means "fairy woman". They are both species names, their difference is that Cailleach is a species with only one member, so she becomes the Cailleach, just like the Grinch.
As David Richerby has pointed out in a comment, "Grinch" and "Peter" are proper nouns, while "dragon" is not.
- If there are many manananggals, don't capitalize the species name.
- If there is only one Manananggal, capitalize its species or personal name.