There's source. A lamia, a minotaur, a sphinx, nymphs for Greek; giants, a dragon, fairy ladies for chivalric romance -- if your readers know such things, but it does lend a certain unity even if they do not. OTOH, very few of them will know why you have dragons no larger than humans with your vampires.
I was thinking of type but really, that would need a back story. All sorts of chimera, from the chimera itself to a mermecoleon that dies of hunger because its ant body can not digest what its lion head eats, would probably mean, for some reason, you had mad wizards /scientists churning them out, probably in some kind war. Overload of undead means either necromancers at war, or a naturally occurring eruption of very evil magic.
Elemental magic might turn on the elemental structure of the world, though. It might not need history to explain why it erupts into elemental beings.