These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Loads and loads of them - in fact, a key part of Angband strategy is just not to get into fights with creatures like these.
Hummerhorns (giant wasps) have a horrifying combination of two abilities; a bite that causes confusion, and the ability to self-replicate. If you have just one of these on your hands and you don't deal with it immediately, you're in for a world of trouble - its confusing bite will hinder you from acting while it happily replicates around you, surrounds you, and bites you to death. Even a high level player can be taken down by these if they haven't taken precautions.
Zephyr hounds come in huge packs and have various breath weapons, which they won't hesitate to blast you with.
Quylthulgs are immobile piles of rotting flesh with various teleportation abilities.
Drolems are the reason you never go below 2,000 feet without poison resistance. They're dragon golems with powerful poisonous bites and breath weapons - and being golems, they're immune to many things that biological dragons wouldn't be.