I have found that in addition to adding a quirk or oddity to a monster the real thing that gets players worried is adding mystery and drama to an encounter. I took a big hint from the 2e Monstrous Compendium Annual, Volume One, Beyond Random Encounters. Describing creatures and even treasures in mysterious, dramatic and moody ways keeps the group guessing and often second guessing themselves, which is priceless to me when running a game. Mundane lizardmen? Not if the players hear and/or smell them first. Everyday orcs? Not from a distance as they howl and gibber in a strange ritual. I have had players scared of goblins in a dungeon because they were dragging something heavy and the group thought it was an ogre or troll limping along. Keep them guessing, make things mysterious and weird. Use their other senses.