Madeline L'Engle's ''Time'' series is chock full of surreal and scary beings. ''NightmareFuel/AWrinkleInTime'' gets its own page, but it doesn't stop there:
* ''A Wind in the Door'': The Echthroi, intangible beings whose goal is to "X" (completely remove from existence) everything and everybody. If need be, they can perfectly impersonate someone, the disguise only penetrable by those who can summon the deepest feelings of true love for the person.
** Also from ''Wind'', The disease killing Charles Wallace, mitochronditis.
*** While mitochondritis itself doesn't exist, mitochondrial myopathies--[[http://www.medicinenet.com/mitochondrial_disease/page2.htm a group of neuromuscular diseases caused by damage to the mitochondria]]--do exist. And yes, they can kill you. Charles Wallace's illness bears a resemblance to [[http://ghr.nlm.nih.gov/condition/mitochondrial-encephalomyopathy-lactic-acidosis-and-stroke-like-episodes MELAS, or mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes]]. Sleep well.
* ''A Swiftly Tilting Planet'': The Echthroi again, this time with the ability to send time travellers careening off course into Projections, their own visions of the desolate, violence filled places they're trying to turn every planet into.
* ''Many Waters'': The Manticores, with a lion's body, a scorpion's tail, and a giant human face that continually shout "Hungry!"