Monsters. Fantasy is full of them, and most of the earliest stories and myths seem to feature them in some form. Whether it’s the creatures of the Greek myths, Beowulf fighting Grendel, or the folk tales of the Guytrash on the Yorkshire Moors (a fairly generic large dog type spirit), non-human creatures seem to give us a sense of adventure, as well as a metaphor for the struggle against our own more bestial elements.
For me, the danger comes when monsters are overused and become over familiar. Hands up, who isn’t scared of the daleks anymore? They’ve been in so many episodes of Doctor Who that we almost know them too well. If you’re doing the same monsters as everyone else, then there’s a danger that people will just go ‘Oh, another vampire. Ho hum.’ Or they would if people spoke like that (Hmm, did anyone else have the thought ‘Bertie Wooster: Vampire Slayer’ then? No?)
The other danger comes when the monsters don’t really mean anything. Most of the Greek monsters seem to have been metaphors for one thing or another, while vampires have famously been used to represent all kinds of things by different authors and werewolves are an obvious image for our own more violent impulses. The problem comes when you start using them just because you think they look cool, at which point they cease to represent anything, and rob your story of a whole layer of meaning.
Take the Figments who show up in my novel Court of Dreams. They’re basically a plot device to allow me to put in anything I want, because they can be anything, but actually, they also work to let me put in a few thoughts on existence and identity that I couldn’t have put in otherwise. Also jokes about bartenders, but that’s not the point.