Man, I feel like we're hitting the doldrums of this challenge.
My one word response: giant spiders.
Most animals and vermin just aren't scary in D&D. Years of them being used as low-level fodder, I believe, have dulled PCs senses toward such creatures. For example, a group of 1st level characters enters a dungeon-what do they encounter besides the kobolds? Giant rats. And at second level they might be able to handle the large centipedes.
Even animals--except maybe wolves--don't get much respect. Over the summer one of my players threw a rock at a wild boar just for fun. The boar killed two characters. But that didn't stop the players from eagerly attacking a giant badger later on.
Giant spiders, however, are universally scary. When players encounter them, they often think: poison! Spiders, even small ones, are just so damn creepy and possess such a foreign and malign intelligence--every aspect of their nature its to hunt and trap prey. Vampires are scary because they look human and suck blood. Giant spiders are terrifying because they can paralyze their victim, wrap them up for later while their organs dissolve into a goo the giant spider can easily suck out.
Giant snakes and scorpions may also strike fear into the hearts of PCs, but they don't compare to giant spiders. Snakes are just crawlling esophagusus. Scorpions are kinda creepy, but not as creepy as spiders despite having stingers and pincers.
Now here's a video of a tarantula goliath.