The ice monster, a first level monster that has a ranged paralysis attack that can go through intervening monsters and hit you. Usually manageable, which makes them Goddamned Bats instead of Demonic Spiders, but beware; a hobgoblin supported by an ice monster can easily kill a low-level player, often without any chance to retaliate or escape. It's even worse when you find a secret room full of treasure and monsters at the end of a straight tunnel...
The rattlesnake reduces your strength, which makes you cause less damage. And they are extremely common—far more so than the ring of retain strength or potions of strength or regain strength. This means that it's very hard to reach the higher levels without ending up enormously understrength.
The aquator rusts any metal armor, reducing its armor class repeatedly until it becomes worthless. And it is also extremely common — far more so than the ring that protects your armor, or scrolls of enchant armor. This means that your best shot at good, durable armor is enchanted leather armor.
Diablo has literal bats in it. They teleport. Diablo II adds Flayers, Maggot Young, Flesh Beasts, Leapers and Imps. The Imps teleport. Both games have Fallen/Carvers. Flayers, Fallen, and Carvers all tend to swarm and can be resurrected by their respective Shamans (except for the undead Bone Flayers, which explode for a nasty chunk of damage when they die). Sand Maggots and Flesh Beasts spawn Maggot Young and Flesh Spawn, respectively. Imps are spawned by huts. Leapers move very fast and jump all the time which makes them difficult to kill due more to being hard to click on than having lots of armor or HP.
Swarms of enemy Archers.
Blood Hawks are literal Goddamned Bats spawned by nests. There are also mummies spawned by sarcophogi, greater mummies that can resurrect almost any type of undead, and Putrid Defilers, which enchant other monsters to spawn Pain Worms upon death. Oh, and all those monsters that can be resurrected? You don't get more experience for killing them again.
Diablo III shows every sign of continuing the trend, with enemies that swell up and explode when killed, spewing out other, smaller enemies like some kind of clown car of the damned.
In the popular — well, as popular as they get — roguelike Dungeon Crawl, Giant Bats have a penchant for not doing any significant damage, but running away just barely faster than you can chase them in most cases, and moving erratically when you try to attack.
There is also a randomly appearing vault full of different flavors of bats. The actual vault name is the source code is "goddamned_bats".
Imps, which will barely give you a chance to swing at them before they teleport somewhere outside your range of vision, forcing you to wait while they navigate back to you and do the same thing again. And they have the fast regeneration trait. But at least they don't do much damage... unless you're unfortunate enough to let them find a weapon on the ground, or worse yet a ranged weapon on the ground.
Unseen Horrors. They do not fly, but are (as name suggests) invisible. They have pretty strong attack (easily stopped by good armor, but deadly against mages, stalkers and giants), speed of a bat and on top of that they flee when heavily hurt. Their erratic movement pattern means that you can't just find it and blast it to death, unless you are standing in a corridor.
Ugly things and slime creatures. While both are fairly weak, they both have a tendency to swarm in numbers- and an even more annoying tendency to run away once they start taking damage. Slime creatures are especially bad about that- oftentimes an injured slime creature will run out of your field of vision and reappear just seconds later, nearly fully healed.
New players quickly learn to fear the common dungeon Adder. Found as early as the very first floor, it moves faster than you, has high evasion and most importantly poisons you, possibly several times, before you are likely to have identified curing potions. A single adder has a good chance of killing a level 1 or 2 character by itself if you're not prepared.
While most of the Goddamned Bats in NetHack are actually Demonic Spiders, the floating eye is harmless in and of itself — but if you bump into (attack) one, you're likely to get paralysed long enough to be killed by some ignominiously weak enemy such as a newt. When a floating eye is blocking your passage ahead, it can be immensely frustrating trying to kill it if you don't have enough spells or ranged weapons yet. Ironically, the corpse it may or may not leave behind is extremely useful— mainly since it allows you to fight floating eyes and not be affected!
In Alpha Man, Grey Mold resists 1/3 of physical damage, making it nearly impossible to kill with normal weapons until halfway through the game. The only choice is to use a special weapon, like a phaser, blaster, or flamethrower on it, or fight until you're almost dead, run away, then re-fight, until the mold dies or you accidentally die.
In the Elona, There's Goddamned Bats and Goddamned Hounds. The only bat that might kill you is the bloodsucking vampire bat, unless you have severely low health (i.e. you just started.) The bloodsucking attack is a 100% hit attack, fortunately if you feel like wasting a feat slot you can utilize it yourself, or start learning some good magic and get magic missile. One shot usually takes them down. However the bats' TRUE purpose is to screw you badly in farming quests where you will be constantly interrupted by offscreen bats while trying to pick the produce, as they flit in, strike, and wander away erratically. Even being interrupted once is enough to fail the 100+ weight missions, as you use up the entire amount of turns normally needed but don't manage to pick anything.
Imps are of a lesser nuisance, they are like slower magic-casting bats, however they're easier to kill unless they are Boss or Arena versions of themselves. You may get killed once or twice by a nether imp's magic, but as they are not as fast you can just find an object to hide behind and naturally heal, then pop around the corner and stab them until they die/blink away, finishing with projectiles. Hounds have elemental breath attacks that, while not very damaging unless you're weak to that element, are fast enough to do it a couple times in a row to slower characters, and nether, illusion, and sound hounds afflict you with status ailments on top of that, which allows them even more free hits. They also have a VERY wide range and the breath may go around 'open air' corners, so you may even die if you distract them with a pet or an NPC. Thankfully drakes and dragons only breathe fire, ice, or lightning.
Bells and quicklings. They're both Metal Slimes combined with Bats. Both are rare spawns however, which may put quicklings more onto the level of Boss in Mook Clothing. Especially since quicklings may spawn with artifact/Great! items as ammunition or projectile weaponry. Bells barely attack, but the reward for killing them is a giant bonus. Silver bells give small medals and platinum coins, and gold bells can give anywhere from 20k to 100k in gold. They have very high magic resist as well as insane speed.
In the multiplayer graphical roguelike game Crossfire, most monsters spawn from generators — but a few, notably mice and centipedes, can simply multiply — if not exterminated rapidly, they can fill the entire dungeon level. The literal bats probably qualify.
Ancient Domains of Mystery introduces giant ants in the first dungeon, which, because of their high natural armour and your low level, will almost certainly not be scratched by almost any weapon you might have or find. If you ever get the message "You hear clicking sounds", run for the fucking stairs. And the quest that unlocks the dungeon ensures that you will be at a low level — rescuing a puppy before it dies of starvation. THEN escorting it back to the surface, back through the ant room. No wonder the puppy's nicknamedKenny.
ADOM has a late-game enemy, the Cat Lord, which will be friendly and give you an amazing artifact if you have never killed any cats, but is extremely lethal if you have. This means that cave lions, cave tigers, and wild cats are very annoying to deal with (requiring specialized non-lethal tactics) and an unlucky early-game spawn can ruin your chances at the Ring of the Master Cat.
In platformer Roguelike Spelunky, just about every enemy moves fast enough to be a bat, except the snakes, and ironically, the bats. The spiders (not demonic, just regular kind) in the first area in particular jump extremely erratically, much like the La-Mulana bats.
Frogs stand still until you get near them, then jump towards you in a completely random manner. In addition to that, the main character's whip is too short and slow to be actually useful against this kind of enemy. And there's a variety that explode.
Bats, while slow, have a tendency to fly just above your whip, which means you have to jump to kill them. When you realize this, they are usually too close to allow you to do that.
Pokemon Mystery Dungeon Explorers of Time (And Explorers of Darkness) feature two lines of Pokemon that explode upon defeat, and said explosion cuts your health in half (unless said victim is a fire type). Stunky and Skuntank don't do much else, but Drifloon and Drifblim have the ability to attack twice when they're not holding an item. And enemies in these games don't spawn items. And they can learn the all-room-hitting, stat-and-speed-raising Ominous Wind...
Ghost-types in general are annoying. Besides far too many of them knowing Ominous Wind, they can phase through walls and attack you while inside those walls, and a good percentage of your moves can't hit them while they're there.
In the Atlus Roguelike Baroque, the enemy Sin Monis has a high defense, and can suck away at both your Health and Vitality. There are floors with millions of the things.
If you end up trying to kill Glues without a sword (which happens pretty damn often, since they only show up at the beginning of the tower) they can be a pain. Unless you just want to step on them, but then No Rewards For You.
Gliros also qualify. These grinning red bastards like to steal an item at random (and no, your equipped items are not exempt), then scamper off at top speed. If you manage to catch up to the Gliro, it might even throw the stolen item at you—which not only causes damage if it hits, but also prevents you from reclaiming the item!
Plus, the game has two different Goddamned Bats of the "inescapable" variety:
The Bubugel, which completely blends in with the walls until you wake it up by getting too close or hitting it. Strays into Demonic Spider territory with its habits of a) smacking you in mid-combo, b) taking forever to kill, and c) falling on top of you when it finally dies. ...Not to mention Nightmare Fuel.
The Sun, which has three or four different attacks that all cause the Lethargy status effect, making you move and attack at a painfully slow speed. Plus, once you cut it down from its corridor-blocking webbing, it will scuttle away, forcing you to chase it down (and subject yourself to more Lethargy) if you want to kill it for good.
Zephyr hounds in Angband appear in groups and almost every kind breathes some sort of element, maybe an Infinity+1 Element. Many players make a habit of using scrolls/spells of genocide on 'Z'.
There are also the Blink Dogs, which also come in packs and can randomly either summon the player to them or teleport themselves away. You'll feel like a ping pong ball if you ever run into them, and you will. A lot.
And then there are the Goddamned Rats and Lice, Explosive Breeders which reproduce with no difficulty or penalty except for an Arbitrary Headcount Limit. If you wake one up and don't immediately kill it, you will quickly have several rooms and all connecting hallways blocked off by swarms of rats that are absolutely impossible to completely exterminate without a genocide spell. If you're stuck in the middle of that swarm, you're not likely to manage to hack your way out unless they literally deal no damage to you. Some variants add a monster called "Cheerful Leprechaun", which multiplies at the same rate and also steals your money.
Lynch Mob Members in the Angband variant Steamband deserve special mention, as they spawn with weaponry, thereby doing decent amounts of damage, they can armor pierce, have above average health, evasion, and defense, they bring their own lightsources (torches) thereby alerting other monsters, and yet STILL SPAWN LIKE MICE. Needless to say, if you first encounter one not in a corridor but an open room, get the HELL out of that dungeon floor immediately. The only surefire way to deal with them is to flee around them, closing all doors, then go down the last remaining open one and kill them on the door itself. Depending on how many times you avoided them while running, you may or may not need a batch of potions to deal with them. Even then, if you aren't killing them fast enough you'll eventually be overran as the critical hits will eventually take their toll on your regenerative abilities.
Shadow Oozes grab you (limiting your ability to attack mostly to punches), teleport away to hide as soon as you hit them, and regain health when they attack you.
Bogwarts can destroy your organic equipment, including your torches and your backpack (which you need to carry all the stuff you pick up).
Krenshars can give you the Frightened status effect, which makes it impossible to attack or move closer to them.
Zombies are incredibly tough and immune to many abilities.
Kobolds and their ilk usually come in packs and often throw tanglefoot bags, which explode into an area which is very difficult to move around in.
Dungeon Maker II has bats, but this trope exemplified with another common enemy, bugs: They always appear in large quantities, and due to the controls will disable you from opening all nearby doors and greatly disrupt your ability to target any enemies that pose an actual threat. Coincidentally they are the only enemy that are impossible to hit up close using a bow. The upgrade, poison bugs, have a chance to poison with every hit and never give any money or items. One hard boss summons them in just enough frequency to take full advantage of these frustrating attributes.
Doom The Roguelike has the original' Goddamned Bats a little switched and tinkered around. Imps are a little less annoying, but they're still there. Lost Souls traded their shotgun resistance for bullet resistance, making them a pain for chaingun/pistol builds to deal with. Former Sergeants got promoted to these, too, as due to the way shotguns work they never, ever miss, and they knock you back when close. They also swarm. A lot.
In Dungeons of Dredmor, any Animal class monster qualifies as this if you choose the Killer Vegan skill. Having this skill makes all Animal monsters peaceful, but they still block your way, are attracted to you, and you cannot attack them or you will get a large debuff to your stats.