But let me back up and explain what I'm talking about. I'll use D&D as my specific example.
D&D has borrowed monster concepts from a lot of sources. For example, many of the ideas (particularly much of the ideas of elves, dwarves, halflings, and dragons) came from Tolkien, and things like minotaurs and medusas came from Greek myth.
But compare the treatment of Medusa in the original myth to its treatment in D&D. In the myth, Medusa is a badass, and defeating her is a major triumph for Perseus. In D&D, though, 'medusa' doesn't even refer to a single monster--it refers to a whole species of monsters. And it's a monster of medium-high level--it's reasonable for a party that gets to this level to have multiple medusa encounters, and encounters with multiple medusas.
This happens over and over in D&D. Minotaurs, vampires, and dragons particularly come to mind as monsters that were terrifying in their original stories, but become routine in their D&D flavors.
And this commoditization saps a lot out of the stories of D&D campaigns by comparison with their original stories--instead of telling epics about defeating Medusa, it becomes a boring routine of killing tough but not awe-inspiring opponents.
Awe-inspiring monsters should not come in six packs.
I blame this on D&D originally, but it shows up in other things based on D&D. In the computer game Angband, for example, it's more or less normal to fight dozens of Great Red Wyrms and Emperor Liches in the course of the game.
Another horrible example of this is the Dungeons and Dragons movie, in which there are so many dragons fighting on each side that they have no more individual grandeur than pigeons. That sort of presentation makes it completely impossible to have any appreciation for the real impact that dragons ought to have.
The question, then: how to react to that in the D&D game I'm running and make good stories possible. One part of the answer: make up monsters of my own, so there won't be this sense of monsters coming in packs that ought not to.