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.
Several things are there to ruin your day, including slimes that split into two whenever you hit them. Then there's the Doomguard, which is a boss monster that splits into two whenever you hit it. On normal difficulty, it's just a very long fight that saps every resource a party has. On higher difficulties, Save Scumming is highly recommended.
Chitrachs deserve special mention, not because they're all that tough compared to Doomguards, but because the Eastern Gallery in the fourth game was made into That One Level purely by their presence. (To be more specific, more than two hundred of them were present.) And chitraches tend to be near null bugs, a creature who can render mages/priests useless with the terrible act of... being close to them. Yeah.
Expect most monsters in the latter third of the game to have upwards of 30-40% chance of parrying or riposting all incoming damage.
Spraying Shrubs don't attack directly, but spray the entire party with acid every turn. You tend to encounter several at once, so it can take several turns to get rid of them all. Meanwhile, it's easy to die without even realizing how fast you're dying.
Goddamned Bats: The gremlins. They're not tough, not very fast... but their only ranged attack is a confusion ray. And they also run away while everyone is confused. To make it even more frustrating, in the original trilogy, they attack twice a round, almost always come in large groups, have no variation in their strategy, and confusion can make a character skip their turn, so 90% of the fight is just waiting for them to finish their attack animations.
That One Level: Avernum 4's Eastern Gallery manages this entirely through Demonic Spiders, giant insects known as chitrachs. They're fairly hard to hit, they have high HP and damage resistances, and every time you perform a melee attack on them they have a chance of blocking and damaging you. Oh, and there are 227 of them out for your tender flesh. Enjoy!
If you thought the Eastern Gallery was bad, wait until the outside of Khoth's Library at the end of the game, where you get regularly swarmed by chitrachs which 1. Now have three attacks on their turn (two is more than enough to stunlock anyone who isn't wearing the heaviest armor possible) 2. Are highly resistance to any form of magic, in a game where Linear Warriors, Quadratic Wizards rules supreme. 3. When they spawn, they completely surround you, and you have to fight 6+ at a time.
The Spiral Pit in 1/Escape From the Pit. It lulls you into a false sense of security by siccing low to mid level undead at you, but halfway through the dungeon, the lights go out, severely limiting your vision and maneuverability, you have to fight through half a dozen Stalker Wights, and the last fight of the dungeon is against Ghosts that can doover 100 damagein a single attack.
The Slith Fortifications from the same game(s). On paper, it doesn't look so bad, just a bunch of Slith flingers, mages, shaman, warriors, their pet lizards, and their chieftain. The problem? There are a ton of them, moreso than most of the other Slith dungeons you've been to thus far. Not only that, but the map itself is ridiculously small and cramped, meaning that it is very, very easy to aggro literally the entire fort if you don't know what you're doing.
That One Attack: You will learn to dread enemy mages' Daze and Lightning spells.