My favorite "monsters" are a mash-up version based in the supernatural factions of World of Darkness: clans, tribes, mage traditions, guilds...because they are "evil" but not simple sociopaths dumbs (yes, gnolls, I am pointing to you!). I would rather villains with potential to become anti-villains, who make serious efforts to be coherent with their own codes.
Some monsters are based in famouse franchises from videogames and movies, but also into "urban legends" and conspirancy theories. Let's imagine Matt "Mad" Murddok (A-Team) talking with Fox Mulder(X-Files) about theories found in internet.
Dragons are cool, but if you add too many predator monsters in the ecosystem then the preys would disappear, and also other hunter species. In the nature some times you could find an animal, from a far zone, not like in videogames where you find and kills hundres in only one day.