... always bad news - especially when they hunted in pairs (or more).
Actually, I often found NPCs to be better at conjuring up fear/loathing than the beasties - especially where the relationship between the NPC nemesis and PCs had time to mature over the course of a campaign.
Ok I know the list is probably endless but I'll start us off.
Most undead creatures were to be feared due to the possibility of being level drained. I used to hate shambling mounds too as they were immune to most magic and if they hit you with both appendages then you were smothered in 2 - 8 rounds unless it was killed. I mean, the things were immune to fire, lightning caused it to grow and cold was at most half damage and how many people actually had things like Plant Control? Oh and if I remember rightly all weapons did half damage too! Where's the weedkiller...............
I hated Cockatrice too as although they didn't do much damage there was always that fear of failing your saving throw and being turned to stone.