The creator of a fantastic fiction universe gets to set the rules for how the stock fictional creatures and peoples work. There are a bunch of mythological creatures that are, as a whole, vaguely defined, and even though their recognizable traits (blood-sucking, flying, fiery breath etc.) are widely known, they need to be cleared up and re-defined to be able to appear as actual characters in a particular work. However, the audience is going to want said creator to stick pretty close to the rules they pick
A writer uses this trope any time they want to use a famous critter or race without having to deal with the all of the various baggage said critter has accumulated in folklore and pop culture over the years, allowing them to leverage existing tropes and characteristics only as much as they want to. So in the end, when they use a term like "elf" or "vampire", we have a pretty good idea what to expect, but at the same time we know that there may be something off about it compared to somebody else's book.
creators often engage in this while populating their worlds, rulebooks and monster manuals. As well as getting the various creatures to fit properly in their particular fantasy scenario and play nice with their game mechanic, they often want to produce as big and varied a bestiary as they can. So, for example, if two folkloric creatures are so similar as to be interchangeable, one or both may end up being altered just to make them distinct and justify separate entries. A similar thing can happen if a creature is known by more than one traditional name. In some cases, a single creature will be spun off into several varieties, often distinguished by colour
A violation of Genre Consistency
, but (usually) not of Internal Consistency
For an index of all fictional creatures see the imaginatively named Index of Fictional Creatures
. See also Stock Monster Symbolism