Terrifying Foe

How scary a monster is, is partially determined by their Challenge Rating. Ostensibly, this should be the sole factor: how threatened you feel is based on how much of a threat they are. But a high CR monster that can devastate the party might not really be scary if it looks goofy or is a funky concept. A pixie barbarian for example. But if given an accurate CR, it might still mess up the party.