I recently finished writing a short story full of undines (water elementals), so I've still got nixes, kelpies, fossegrimen, and an antagonistic fire spirit on the brain.
There are also some versions of elves and dark elves out there I'm quite fond of, and Ven Polypheme the Nain (the titular protagonist of a series of journals by Elizabeth Haydon), not to mention that obscure creature you've probably never heard of...what are they called again?...oh yes -- the vampire. ...Preferably not the kind with skin falling off and teeth so big, their own mouths are bleeding.
End of the day, though, I think the coolest kind of fantasy creature is the kind that doesn't just have power, it can have power over you.
i hate that there are like fifteen different ways to spell it though. 
