Clifford Simak/YMMV


 * Designated Villain: The Wheelers in The Goblin Reservation are introduced as a phenomenally ugly species, and there are vague but unspecific rumors of atrocities they've committed against human colonists, but it's never made entirely clear why they are, as the protagonist tells us at some length, necessarily enemies of humanity. Toward the end, they do attempt to kill the protagonists with poison gas (and it's revealed they did murder a person that way at the beginning of the book).