"Frankly, it's much more common for pop culture staples like these to have lots of internal variation than for them to have some kind of rigid canon defining how everything works."

— Red, Overly Sarcastic Productions: "Halloween Special: Werewolves"

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.

Fantasy RPG 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 or habitat.

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 (including Angelic Tropes, Dragon Tropes, the Undead Index, et cetera). See also Stock Monster Symbolism. See also Our Humans Are Different.

Tropes:

• All Trolls Are Different
• Our Alebrijes Are Different
• Our Angels Are Different
• Our Archons Are Different
• Our Asuras Are Different
• Our Banshees Are Louder
• Our Centaurs Are Different
• Our Cryptids Are More Mysterious
• Our Dark Elves Are Different
• Our Demons Are Different
• Our Dragons Are Different
• Our Dwarves Are All the Same
• Our Elves Are Different
• Our Fairies Are Different
• Our Gargoyles Rock
• Our Genies Are Different
• Our Ghosts Are Different
• Our Ghouls Are Creepier
• Our Giants Are Bigger
• Our Gnolls Are Different
• Our Gnomes Are Weirder
• Our Goblins Are Different
• Our Gods Are Different
• Our Gryphons Are Different
• Our Hippocamps Are Different
• Our Homunculi Are Different
• Our Hydras Are Different
• Our Imps Are Different
• Our Kelpies Are Different
• Our Kobolds Are Different
• Our Liches Are Different
• Our Mages Are Different
• Our Manticores Are Spinier
• Our Mermaids Are Different
• Our Minotaurs Are Different
• Our Monsters Are Weird
• Our Nymphs Are Different
• Our Ogres Are Hungrier
• Our Orcs Are Different
• Our Perytons Are Different
• Our Pixies Are Different
• Our Sirens Are Different
• Our Slashers Are Different
• Our Sphinxes Are Different
• Our Spirits Are Different
• Our Titans Are Different
• Our Vampires Are Different
• Our Werebeasts Are Different
• Our Werewolves Are Different
• Our Wights Are Different
• Our Witches Are Different
• Our Wyverns Are Different
• Our Yaoguai Are Different
• Our Zombies Are Different
• Reverse Supernatural Creature

Examples: