Geckos in Fallout 2 and Fallout: New Vegas (though Fire Geckos, Golden Geckos and Green Geckos can be Demonic Spiders depending on your level). They are easy to kill, but they are very fast, will try to swarm you, and the damn things are everywhere. Thankfully, a few of them tend to be nonhostile unless provoked.
You'll be encountering raiders throughout every game; they're everywhere, they're aggressive and they're always hostile. But they never switch away from light equipment, so they turn into Goddamned Bats as soon as you get some XP and basic armor. At higher levels they're essentially target practice, but you can never ignore them because they'll start combat turns and force you to engage them or flee.
Bark Scorpions. They are fast, tend to come in groups of two or more, and will actively try to flank you. They will jump, either to close the distance or for a quick escape, and their poison is horrifyingly potent (and causes an Interface Screw to boot). The only thing keeping them from joining their radscorpion cousins in another trope altogether is that they don't hit particularly hard, nor are they anywhere near as tough. Still, if you are low level, and you get stung with no antivenom on you, you are pretty much dead.
There's also those damn Nightstalkers. They aren't particularly dangerous, unless you have the misfortune to stumble across a pack of them, but they're fast, difficult to hit, and make a horrible hissing noise.Later on, they become invisible. The only saving grace they have is that their blood, tails, and eggs are worth a lot of money. Their brethren from Old World Blues, however, are straight up Demonic Spiders, partly because of their Level Scaling.
Bloatflies, giant mantises, and giant ants are generally nothing more than minor annoyances, unless there are a lot of them. The Zion Mantises from Honest Hearts, however, are upgraded to Demonic Spiders.
Dark Chronicle has Gemrons, little baby dragon enemies with different elements. At a distance, they stun you with fireballs; at close range, they roll towards you and knock you down... and then they spit fireballs a split second after that. The regular bats aren't nearly as annoying.
... unless you're going for a "Defeat Enemies With Items" medal, at which points you'll be damning those high-flying, nigh-unreachable bats quite often. And loudly.
Or a "Defeat all enemies using only Max's right-hand weapon" medal. Max's weapons can deflect fireballs, and they count as a magic/gun attack. The only GOOD thing one can say about Gemrons is that once you get the Gemron badge for Monica, and level up her monster forms, you can have a LOT of fun flying around dungeons in Gemron form blasting enemies away with fireballs.
Nox had some truly frightening bats, especially for players without area-effect attacks. In online play, several Conjurers could band together with four bats each, creating a Warrior-wasting bat-tlegroup.
Dragon Quest and other games of its type pretty much burns an unpleasant association with the phrase "Slime Summons Reinforcements!" The worst offender of this was Shining the Holy Ark, where you could start with enemies A, B, and C and have cycled through the entire alphabet before killing the last one.
And don't forget the Bell class of monsters, who appear in large groups, summon more bells and other monsters...and certain types of Bells, once they've gotten 8 or so of them on-screen, either play a level-up song or an instant-kill song...
In the NES games at least, with insane random encounter rates, EVERYTHING was either a Goddamned Bat or a Demonic Spider.
The cliff racers in The Elder Scrolls III: Morrowind. Annoying because 1) they're everywhere; 2) they can keep up with you even if you max out your speed, acrobatics and athletics stats; 3) you can't sleep when a cliffracer is near; 4) it's nigh-impossible to fight them in melee unless they are facing you squarely. If you played an archer, though, cliffracers made wonderful pincushions - once you grow accustomed to the wrong hitbox, which adds the annoyance of having to shoot at their legs to hit their torsos.
As if the cliff racers weren't bad enough as is, there is a bug that can turn them into Demonic Spiders easily. If you happen to be running away from one, and you aren't fast enough to get away from it (without speed enchantments, you won't be), it will attempt to attack you as you run. Depending on the terrain, a bug can allow it to land attacks faster than a machine gun, which can shred low-to-medium level characters in a split second, cripple medium-to-high level characters, and still piss off god-level characters.
Slaughterfish are an aquatic version of this trope.
There are more wolves in The Elder Scrolls V: Skyrim than have ever existed in the history of the planet Earth, and after a few hours of exploring, you will have littered the landscape with their corpses. The fact that their pelts aren't worth very much only makes them more irritating.
Skeevers, which are basically giant rats, are barely more than a bump in the road for even a level 1 character. What turns them into Goddamned Bats, however, is the ridiculously high chance they have to give you a disease.
The dragons themselves, by about the point you're attacked every single time you exit a town, are just challenging enough that you seriously consider missing out on the open world you bought the game for in the first place and just fast-traveling everywhere. What's worse, though, is that dragons drop valuable but heavy loot upon death. If you're overencumbered, you can't fast travel; enjoy that walk to the nearest merchant.
You've killed the Dragon and you're ready to just fast travel to your next destination instead of risking more dragons. But not so fast, you're still in combat, but with what? Mudcrabs. Any time you're anywhere near water, they're there. They burrow themselves into the mud and look like rocks so you never know exactly where they are, even when they're coming to attack you. As already stated, they have a tendency to lock you into combat completely unexpectedly, which disables your fast travel.
Slaughterfish are even worse, as not only do they disable fast travel like the mudcrabs do, but they're nearly impossible to track down and kill since they're in the water, and underwater combat is virtually impossible. You've either got to somehow blast one from the shore where they're impossible to see, or just run in a random direction (hopefully away from the slaughterfish) until the game lets you fast travel again.
Lets talk about The Elder Scrolls IV: Oblivion Got a hundred percent chameleon? That's well and good against 99% of enemies so you confidently stroll through enemies and pat them on the head, then step into the planes of oblivion and walk into the inconspicuous razor blade land mines.
Some of those damned archers and mages in Dragon Age: Origins. It doesn't help that the difficulty is rather inconsistent, but generally, these two are the most annoying types of enemies you'll face. (Friggin FIREBALL!!!)
There's a few fights towards the end of the game where you have to fight groups with archers using Scattershot, and a ton of guys with shields who spam shield pummel. Yeah. Those are fun.
Dragon Age: Inquisition has despair demons. Per their name, they tend to make players despair of ever enjoying the game. They don't do nearly as much damage as terrors, revenants, arcane horrors, or pride demons, but they love to spam ice magic that slows, staggers, and/or stuns their targets, and fly away at high speed whenever anyone gets close to them. Alongside stronger enemies, they can make an encounter substantially more dangerous by locking down characters while their allies move in for the kill. By themselves they're no serious threat, but still drag out encounters to nigh-interminable lengths, especially for melee-heavy parties.
Whenever you're trying to hunt a Hyren in Magi-Nation, you'll run into these.
The SNES game SoulBlazer has two varieties of bat-form Goddamned Bats that try to circle you in swarms just beyond your sword-reach and swerve to strike if you ignore them and try to walk normally. And they can fly through the walls.
They however become much less of a problem once you get a certain low cost rotating spheres spell that suspiciously conveniently circle around you at about the same range that the bats circle you.
Soul Blazer is also one of the rare games where one might be incited to proclaim "Butterflies!''"
Heck, the game is full of non-bat Goddamned Bats. You've got jumping caterpillars — in fact, several types that only move into your sword range once you get in a spot where they're gonna hit you. You've got giant flowers that put out tiny bees that you practically cannot kill until they get you — very hard if you're trying for a no-damage run. There's dragons that leap out of the water at you with no visible sign that that's an area to watch out for. And then there are the non-spawned guys whose only purpose is to make killing the spawned guys that much harder, and who are unkilled whenever you return from freeing another citizen — the pillars of fire that spit fireballs at you are slightly annoying, but the freaky face statues that conjure up icicle death in your path, that's somewhat more than merely annoying (because you would be perfectly safe from the forewarned icicles conjured 2 spaces away from you... if you weren't trying to lunge at bats and sorcerers).
Quest for Glory IV had an enemy which combined the Demonic Spiders trope and this trope. BADDERS. Spider-Bats with poisoneous attacks which flew at you one at a time so you could only attack them when they were swooping at you, so that meant that the only time you could make an defense was when THEY were making an OFFENSE.
This is counterbalanced by the fact that, unless you're a fighter, there's no automatic Badder fight at the beginning of the game. Also, you're given direction on where to go, the forest you're stuck in is not all that big and poison both works very slowly (it will take an hour of real time to kill you, while traversing the forest takes about five minutes if you don't know where you're going) and eventually heals itself for weaker forms of poison. Finally, the enemy encounter rate is turned way down until you reach the town for the first time (in fact, the only way you can possibly get into a fight is to go looking for one, that is, straying between screens for a long time), and after about five minutes of real time, the sun comes up.
In Quest for Glory V, you have the badders' long lost cousin, the batties. No poison this time since they're bats, but their small size, high speed and large numbers make them annoying as hell. Worse, they only show up at night or in dark places, making them very difficult to spot until it's too late.
Neoshadows, Darkballs, and Defenders in Kingdom Hearts: Chain of Memories. All of them are fond of using high-level cards (8s or 9s) that will stun you if you try to hit them with a lower-level card. The former two are also prone to chase you, which can lead to you running away while you look for a card that will stop them. Neoshadows are particularly annoying, both because they travel in swarms (so you end up getting repeatedly pounded after a single card-break), and because they can flatten themselves against the ground to avoid your sleights (normal Shadows can do this, too, but they're much weaker).
Kingdom Hearts II provides you with various Nobodies; such as the Berserkers and Dancers, both of don't kill them fast enough.
Still in II are those fucking undead pirates. They are rather easy on their own, but what makes them annoying is that they are invincible to any attacks unless they are in moonlight. It's incredibly easy to accidentally knock them right out of the moonlight mid-combo, and some areas have a lot of shady spots, meaning you'll be constantly pushing them out of the moonlight, forcing you to wait until they step out of the shade. The Japanese and Final Mix versions are even worse in this regard, as fire attacks set them ablaze and cause them to run around erratically, a trait that was censored in the US and European versions.
Kingdom Hearts: 358/2 Days manages to be even more annoying than the enemies listed above, with the Emerald Serenade Heartless. It's incapable of attacking and only being able to move on a set path.... But it always runs away, is rather fast, and has a boatload of HP, making it an extremely annoying enemy to kill. Oh, and at least two missions require you to kill it. One of them is in a fucking maze.
The Emerald Serenade is part of a family of Heartless with a name scheme involving a colour followed by something related to music (for example Red Nocturne, Yellow Tango and Blue Rhapsody). They are all small, flying, show up in groups alongside other Heartless, can hit you from a distance and in some games can inflict statuses as well. The ones introduced in Days deserve special mention; there's ones that teleport when you hit them, ones that teleport you when their attacks hit, one that causes random status effects, one that emits a poison that can EASILY kill you if you try to attack it. They all fly too. And they all appear in the aforementioned FUCKING MAZE.
Birth by Sleep continues the long and glorious tradition with the Mandrake Unversed. At long-range, they'll constantly pelt you with wind slashes which can stun-lock you while you're dealing with its buddies. A couple of these guys can keep you stun-locked practically forever. And when you try to close in on them they'll either poison or confuse you.
If you think about it, the majority of the game's enemies are designed with this trope in mind. Scrappers are designed to hang back & attack during lulls in combos; Shoegazers become invincible for a period if not beaten quickly enough; Spiderchests burrow and poison you; Archravens fly high in the air, grab treasure prizes, and attack anything they touch when doing so; Blue Sea Salts can freeze you; Chrono Twisters cast stop; Axe Flappers attack with projectiles & feign being stunned (also, they're batlike). Oh, and of course, being Kingdom Hearts enemies, they swarm you like crazy.
Hell, this dates all the way back to the first Kingdom Hearts as well. The Large Bodies (which have appeared in just about every game) may very well qualify, because they block any blow you deal from the front with their disgustingly incredible fat, so you have to strike them from behind. Depending on which game you're playing, this can either be laughably easy or a gigantic pain in the posterior (like in 358/2). The aforementioned Darkballs were also just as annoying as they were in CoM. At one point, you have to fight an entire group of them (during the Final Boss, appropriately enough). Then there were those damned monkeys that ran circles around you and could very much overwhelm you early on, especially on Expert Mode. And the Nobodies mentioned above? Not the only Bats in KH2. There were the freaking flying zombies that chased Sora everywhere, the Crimson Jazzes that are literally impossible to run away from, even with maximum movement abilities, and surround you with exploding fireballs, the Toy Soldiers and Graveyards, the Morning Stars, the... you know what? Let's just say "Kingdom Hearts is the best example for Goddamned Bats" and leave it at that.
Practically every Nightmare in Kingdom Hearts 3D has something about them that can make them one of these, whether it's spamming a move that inflicts Standard Status Effects, randomly breaking out of your combos, and/or keeping you staggered with multi-hitting attacks, on top of being a heck of a lot more aggressive then enemies of the previous games in general. It's far easier to name the ones that don't have some manner of annoying quality to them. On the plus side, said annoying qualities can be turned against them once you create their Spirit counterparts to use as party members.
The Imp enemies in Disgaea appear designed to drive players into a rage. They have very long movement and jump ranges, and their Hell Pepper attack deals surprisingly large amounts of damage and poisons survivors.
Imps are also one of two monsters with a healing special, which uses a separate targeting algorithm from attacks. allied/neutral units. Unfortunately, it still counts as an "attack" for the purpose of turning neutral Dark Assembly monsters against you...
The enemy Thief and Kunoichi classes in Disgaea 2 can lean towards this. As no matter how many thousands of levels above them you are, they still land their status moves on you almost 100% of the time while your own tend to fail about half the time. Little is quite as annoying as having a lvl 4000 die because two lvl 10 kunoichi poisoned it and put it to sleep at the same time. Prinny Dance comes close for the same reasons but it actually tends to miss or at least not always inflict an affect if the hits fail to do damage.
Of course, the bright side of this is that almost every unit used against you can be used by you. And yes, it is satisfying to inflict three different status effects on one bigass enemy.
The Imps were replaced by the much less annoying Mothmen in Disgaea 2... only for the Mothmen to become Goddamned Bats themselves in Disgaea 3. Thank goodness we've never seen both of them in the same game, huh?
The Ambling Pirates in Disgaea 2 definitely qualify for the "appearing too often" category. To get to the best endgame areas, you must encounter and defeat 16 different kinds of pirates for their unique map pieces, all of which will appear at random in the Item World (randomly generated levels you can go through to make equipment more powerful.) It's frustrating as hell when you're down to the last one or two kinds and two out of every three encounters is with this guy, especially considering that its more likely to find pirates the more map pieces you have.
The Poison Lilies in Phantasy Star Online. Not only do they spit out the titular poison, they also can paralyze you, making you unable to do anything but use items. Ob Lilies in the Ultimate difficulty level are even worse, spitting out the instant death technique Megid. Oh yeah, and they can still paralyze you.
In fairness, Poison Lilies are a joke once you get to know them. Goading a Lily into trying to peck you but failing is a trivial affair, and while it's doing that you can simply shoot at its friends - the pecking lily will never stop trying and failing, and once its friends are dead you can turn your attention to rapidly attacking the pecking lily before it has chance to paralyse you. In addition, armor upgrade units which prevent paralysis are not difficult to acquire, and poison is probably the least annoying status effect in the game once you reach a decent level.
And then in Episode 4 in Blue Burst there's Zus. Large pre-historic looking birdlike creatures that fly around, dive bomb, and shoot Frickin' Laser Beams from their mouths. Not quite as commonplace as most other Bats but still very annoying. They fly around very fast and are unhittable while they dive bomb you (Unless your clever or have Gifoie), which always knocks you down and sets you up for another hit, or preparation to be mobbed.
Thieves in Wizardry IV. They can steal any item from your bag, even plot-critical ones, and as soon as they successfully steal they run away and then you have to wander around the floor to find them again. They'll even abandon their party if they're fought in a group, again forcing you to spend extra time and keystrokes after the fight. However, they cannot steal out of the Black Box so anything inside it is safe if it gets taken.
Oh, where to begin in Wizardry VII: Crusaders of the Dark Savant...
How about with Fish? Red Piranhas and Piranha Sharks are about the only things you'll meet out on the ocean, come in large numbers, don't give much exp, and hit decently hard.
Or Birds. Spectral Crows blink in and out of existence randomly, meaning magic is unreliable, ditto your heavy melee weapons. They also like to inflict fear on multiple characters, giving them a chance to run away from combat. Dragon Rooks breathe acid on multiple characters, fire crows breathe fireballs, and vampire vultures hit fairly hard, have a lot of hit points, and can drain maximum hit points-from which there is no recovery.
Moths, too. Spectral Moths do the same thing as spectral crows, while other moths enjoy blinding, poisoning, and paralyzing party members.
Ghosts-virtually Metal Slimes. Very resistant to magic, can inflict all the worst Standard Status Effects, can hit hard on multiple characters when they wish, and like to vanish when low on hit points.
G Rattkin. Fast, appear in large numbers, and can usually do a lot of damage before the AoE spells clear them out.
In Wizardry 8, any large gathering of monsters (usually eight or more) can turn into a Goddamned Bats scenario, since fighting them is often extremely time-consuming, and they all get to attack...
Not to mention the fact that if you scare them away they high-tail it into the distance leaving you stuck in combat whilst you try and catch them because you can only get out of turn based play once they die!!!
The Spiteful Crow is another infuriating enemy that was found very early in the game (and twice more after Onett) — they were faster than you (and were quite evasive), and could steal one of your items!
The Territorial Oak could be annoying (exploding trees, anyone?). But at least it usually showed up alone, so you could speed through its death message to end the battle before it killed you.
Another enemy in the same area as the Territorial Oak is the Mobile Sprout, which fits this trope to a T. It can heal with Lifeup, it can suck away your PP with PSI Magnet, and it can create more Sprouts. At this point in the game, you only have one party member, so it can get very annoying fighting them.
More or less any enemy that can inflict status effects. If they can do it, it will very rarely fail and they're surprisingly difficult to get rid of.
The worst are mushroom enemies. They are often fast enough to go before the party gets a chance to kill them and inflict a status effect that can't be gotten rid of without trudging all the way back to town. The "mushroomized" effect combines a chance of randomly attacking a party member with Interface Screw that makes walking back to town to cure it a chore.
In MOTHER 3, "brooms" appear near the end of Chapter 2, at the end of a dungeon filled with enemies that are kind of annoying to kill because you gotta use PK Freeze to kill one at a time. Anyway, the brooms are relatively easy to get by since they move in a pattern, but they lure you in with items! And if they catch you, they sweep you off your feet and you land in a garbage can all the way at the beginning of aforementioned dungeon.
Worse are the Tiny Lil' Ghosts in Earthbound. They can randomly freeze, posess or damage one of your party members each turn, and can only be removed if that member faints or goes to a healer. Fortunately, they only appear when certain ghost enemies try to attack, and can even be killed by the possessed party member.
Mass Effect: Thresher Maws are frustratingly time-consuming and boring to kill. What's particularly pernicious is that, when they appear, they sometimes spawn directly under the Mako, killing you instantly without warning.
Normal Mooks sometimes did this by charging straight at you, even if they had sniper rifles or machine guns. Irritating, as you have to get out of cover and turn round before you can kill them.
Any biotic mook with Warp in the first game. Usually no stronger than their buddies, but they knock you to the floor with a very hard-to-avoid power while their allies fill you full of holes. They also repeatedly shout I WILL DESTROY YOU to put more salt in the wound.
Husks are also annoying and tough to fight in large numbers. In the first game, they love softening you up with an area of effect blast of energy that kills your shields, giving them a distance attack in addition to the old flailing arms. Bad enough when they were in a massed horde, even worse when paired with geth: The former requires you to remain constantly on the move, while the later requires gluing your butt to cover.
In Mass Effect 2, this gets pushed to Demonic Spiders levels when they're accompanied by Scions, for the same reason as the Husk/geth combo. At least now only some of them explode.
It gets worse in 3. Husks can now grapple you, forcing you to mash the melee button to shake them off. This is annoying at the best of times, and fatal when it knocks you out of cover into heavy enemy fire.
Geth hoppers can jump from surface to surface effectively instantly, and do it a lot. Fortunately, they can be targeted with biotics rather easily.
In Mass Effect 2, Harbinger can quickly reach this status. He's not so hard as to be boss caliber, but he's constantly rushing your position, knocking you out of cover, and even if you kill the body he's possessing, he'll just take over another. This form is redundant.
LOKI and FENRIS mechs too - although a few well-placed shots can make them explode on each other.
Guardians in 3. They're only dangerous at very close range, but if you don't have any biotic squad members, a sniper rifle, or anything that can punch through shields, then you'll just be trying to pop them through the slots in their shields or flank them.
Averted for infiltrators, who find them easy prey. Not only are infiltrators good at sniping Guardians through their slots, but Guardians also move slowly.
Mass Effect 3 has one for every type of enemy you can fight against in the multiplayer cooperative mode:
Cerberus has Combat Engineers, who are relatively easy to defeat as they don't understand cover, but they can drop a turret which will eat through your shields like nothing else, and they love to put them at choke points.
Geth have Pyros, who have very short-ranged flamethrowers, but are completely implacable and have lots of resistances due to their Shield/Armor mix (you can't hack them without taking both layers of protection down, for example).
Reapers have the aforementioned Husks. They also have Marauders, who are individually easy as even their shields are pretty pathetic, but when they're around other units, even out of sight, they will boost those units with special armor that you have to shoot through or around in order to injure the unit affected.
And now the Retaliation DLC is set to bring back Abominations (those exploding Husks we mentioned) and Scions. Alien gods help us all.
In Shining Force, the Giant Bats, and their later incarnation, Seabats. Each is a flying enemy with a large movement range and higher Agility than other enemies for that part of the game (meaning your attacks are significantly more likely to miss). The original Giant Bats can even randomly put your characters to sleep with a physical attack, making them skip at least one turn.
Shining the Holy Ark — many, many creatures could call for reinforcements or duplicate themselves in some way, making random encounter battles go on for 10 minutes or more. Bear in mind these aren't boss battles. Saying that, a couple of the bosses had endless mooks appearing to help them out.
The World Ends with You has a few of these. Among them: Goddamned Bats that are fairly difficult to hit in combat, Crows that steal your pins during combat (disabling that attack until the crow is killed off), Porcupines that drop exploding needles on the battlefield, and Jellyfish that, on harder difficulties where you do less damage with attacks, can often be very difficult to kill off-they summon more of themselves, often faster than you can kill them.
Don't forget Elephants that spam ridiculously long-lasting stomp attacks that do no damage but interrupt whatever action you're currently in progress of performing, even if you're floating a good 7 feet off the ground and the actual footprints that spread out and cause the stun effect are several inches in height at best.
No damage if you're not getting hit by the attack itself, you mean. It's freaking painful if you don't avoid them, especially if you're playing at level 1 to boost drop rates. Also, the interrupting "whatever action you're currently in the process of performing" is worse than it sounds because there are a lot of attacks that take more time to ready than the stomps allow for.
And Pigs; while harmless, they run away from battle in seconds unless killed quickly.
And Taboo Noise, which will actively seek you out during a scan.
And Frogs, who latch on to your characters and attack, and will absorb either short- or long-range attacks depending on their color, along with Shrews that plant deadly bombs and launch drill-like attacks at your characters, and burrow underground to make themselves more difficult to hit. The former is especially a pain in the ass because not only does Joshua get knocked out of the air if he is hit, but he has to come back down to the ground to dodge them.
And the Sharks, which eat other noise to make themselves stronger and hide under the ground while you're trying to hit them so you have to guess where the hell they're going to pop up.
The second expansion pack of Neverwinter Nights gives us Duergar, or dark dwarves. Every single one of them has the ability to cast improved invisibility, and every single one does so the moment they realize that you're there. And the worst part? They're the second most common enemy in the campaign. The loading screen tips weren't lying when they said that you were going to die often.
At the epic level ranges where the expansion is located, you can have an overabundance of spells and - even better - artifacts giving you true seeing, rendering their little trick useless to begin with.
Duergar were also in the original campaign. This was a pain because a) an entire region would spawn NOTHING BUT SQUADS OF DUERGAR, and b) if you'd reached a certain level they'd hit you with Phantasmal Killer. What does this do, you ask? Unless you pass your Will and Fort saves, it does Exactly What It Says on the Tin. Having to respawn 10 times per level because the Random Number God leads to a handful of bad saves? Does horrible, horrible things to your money and XP.
In Paper Mario: The Thousand-Year Door, the Crazee Dayzees, which tend to pop up on narrow pathways, making them hard to dodge — and should you succeed in slipping past one, there'll be another a little further along, leaving you trapped between two. In fact, the whole turn-based fighting set-up of the game turns all the minor opponents into potential time-wasting Goddamned Bats — although they are useful for Level Grinding.
Towards the end of the game, you actually run into bats, which swoop down from the ceiling and are very hard to avoid or get an initiative hit on. There's also a chance that one or two bats will spend the resulting battle attacking from the ceiling (instead of hovering like most flying enemies), where Mario can't jump on them. I hope you remembered to equip Hammer Throw or Quake Hammer...
In the first two Paper Mario games, there are ridiculously annoying little creatures called fuzzies. They suck your health, can split in two, and have really hard to time action commands. What's more, they're everywhere and are among the hardest enemies to avoid. Be warned.
However, use of the Zap Tap badge renders them unable to damage you, as it electrocutes enemies that directly touch Mario; the only way Fuzzies can hurt you. But they're still annoying, because they will keep dividing. As soon as you kill one, another will take its place...
The Shroob Rexes in Mario & Luigi: Partners in Time. The irritating thing is that you CAN'T jump over them if they do that "charging at you" attack if they haven't been stomped at least once or twice to squish them down to a more appropriate height; and the worst part is that, with their high attack and speed, you won't be able to strike first nor will you tell which enemy in the overworld WILL have them on their team.
Hermite Crabs (and their recolours). They take at least two hits to take down because of their 'hide under rocks' gimmick, can't be defeated instantly through a pre-emptive attack thanks to the same gimmick... and are ridiculously common in Wakeport and parts of Driftwood Shore. Oh, and god help you if you want to try and beat the Expert Challenges involving them, since you have to also dodge their (extremely annoying) attacks perfectly.
Viruses. They come in huge packs... outside of the Dream World, so the Bros. do not have as many attacks for dealing with large crowds. Their gimmick is like in the game they originated- get three of the same color to touch and they instantly die. However, getting the exact color needed to clear out several of them is difficult and comes down to memorizing which order of color they change when hit. When they attack, they go on huge parades which require you to hit them in just the right order... or else get barraged by a crapton of Viruses that slow, lower defense, or even shrink (basically, weakened attack and you can't use Bros Powers). When their ranks are lowered, they start moving towards the Bros. in a way that the camera just can't show right; get hit and you end up tripped (complete paralysis until hit again). They also have a crapton of defense, so once you get down to a few rows that just aren't in the position to be cleared, have fun whittling away.
Bandits. Their basic attack will steal 100 coins, and if you don't kill them within a few turns, they run away and it's lost. Then they start using decoys that will do huge damage and most certainly dizzy if you hit the fake. They also come in packs of 5 or more. They also have a lot of health.
In the Dream World, there are Spinies, which take an annoyingly long amount of time to kill because, well, they can only be hurt by hammering them then jumping. Eventually they get up the same turn you hammer them, and their only attack takes absolutely forever to go through. If that wasn't enough, sometimes a Lakitu will appear, which can make even more of them appear.
In the earlier Ultima games, Gremlins EPITOMIZE this trope. They're small, attack (usually) in packs, and on their own don't do a lot of damage. Doesn't sound like much, until you realize they steal food from the party. Food in those games served as a counter for how long the party could stay in a area — run out of food, the party starts losing health until they either reach a town or die. So, Gremlins could steal ALL your food — condemning your party to a slow death if you couldn't reach a city in time.
In the turn-based online RPG DragonFable, any mushroom-type monster is ridiculously hard to hit. This gets even worse when they're level-scaled.
The Minx Fairies are incredibly aggravating — if you manage to get them down to less than half of their HP, they'll Full Heal themselves, which costs them only one MP. If you don't stun them or drain their mana so they can't use the move, you're in for a long fight, especially on the higher levels when they have about 200 MP. And worse, they're strong to Light, unlike just about every other monster in the game, making one of the two most powerful weapons of the game useless against them.
Gothic: Minecrawlers. Minecrawlers. They have pretty nasty damage considering the stage of the game where they're first encountered, but wouldn't be nearly as bad if they weren't... well... in the mines. Which means that you're in a confined space, which is occasionally dark, with blind corner after blind corner, plenty of fatal dropoffs, infinite opportunity for them to sneak up behind you and to top it all off nicely, the "helpful" NPCs that would otherwise be looking after you have no idea how to climb ladders and so end up running in circles while the minecrawlers eat your face. Oh yeah, and did we mention they're giant, hissing, screamingants?
In the Might and Magic series, from the point battle started to be open as opposed to having "battle-areas" (like JRPGs), that is, from M&M 6 onwards, any enemy in great quantity could surround your party and start to plummet you. The best skills you can use against them, that is, area damage effects, will also affect your party, and you're down to using less effective multiple projectile spells like Poison Spray or Spark. The worst of the lot, you guessed it, are Goddamned Bats (and Rats), who have the annoying habit of diseasing or poisoning your party members, rapidly deteriorating their stats and health unless you have some pretty advanced Body Spells or alchemic potions (which are fairly wasteful mana-wise, the first, and rare/waste of ingredients for other potions, in the later.)
Seiken Densetsu 3 has the line of "Dark Priest" enemies. All of them are spellcasters who love to spam Healing Light. The Necromancers in the final dungeons are the worst of the lot, responding to everything with at least one casting (usually two) of Healing Light that heals them for over 400 health. They have 800 or so and you can do about 400 with one move if you're lucky, and can maybe hit them with two before they respond with Healing Light if you're really lucky. What's really annoying is that they don't even attack. They just continually cast a spell that drops every aspect of your ability to deal damage, making fights with them nigh unwinnable unless you can clear the effect. Oh, and they usually spawn in rooms that trap you until all enemies are dead.
Insidious Maya in Persona 3 are a prime example. Unlike most other Maya, they don't have any offensive skills. Instead, their skillset was designed to make them as annoying as possible. They know Diarama, which will generally heal themself or an ally to full health, Life Drain, which also keeps them healthy and Eerie Sound, which inflicts Distress on the whole party, making them more vulnerable to critical hits. Which, in Persona 3, grant the enemy an extra turn. This wouldn't be so bad were it not for their tendancy to show up in groups of five. If you get a preemptive strike on them they're not so bad, but if they get one on you things can go From Bad to Worse very quickly. Worst-case secenario: Eerie Sound hits everyone and you're stuck in a constant loop of getting critted, knocked down, then getting back up only to be knocked down again. They're weak to Ice, but when there's so many of them the chances of missing one and ruining your One More chain/chance for an All Out Attack are VERY high. And then they'll just start healing again. Not the toughest of enemies, but among the most irritating.
The Tigers in the second dungeon of Persona 4 spam the an all-party-damaging attack and seem to constantly critical hit your party. On top of that, they take a LONG time to kill. It's possible to spend at LEAST fifteen minutes on one set of two.
The trick to the tigers, at least in their later appearances when you have more levels, is that Yosuke's Tentarafooalways hits them with confusion, which happens to disable enemies quite thoroughly.
Brave Fencer Musashi had some of these too. In underground dungeons there are some bats among the other enemies, who are just IMMENSELY annoying: they move randomly, are pretty hard to hit due to the weird camera and they follow you through the floor (so you can't simply run away from them).
Monster Hunter has the Cephalos and Cephadrome, which swim around in the sand faster than the player's sprinting speed, with only a very small fin exposed for the player to (hopefully) hit. And then there's the Monoblos and its relatives, which have an annoying habit of burrowing into the sand and staying there, thus being undetectable by any normal means. If you lose track of a Monoblos and have no extraordinary means of locating it again, you're hosed. Thankfully there are countermeasures which can be used to somewhat ameliorate the annoyance posed by the above monsters.
And don't forget the Cats. How to grief another player: kick a cat and bugger off sharpish before all its friends show up with bombs.
The Bullfangos are basically boars that travel in groups and will charge you the instant they see you. Getting hit by one will send the hunter flying. Whoever thought of delivery missions with parcels that break if you drop them should be beaten. Whoever thought of Bullfangos should be shot. Whoever thought of putting Bullfangos in delivery missions should be beaten, shot and beaten again for good measure.
Don't forget Vespoids, the flying wasps. Most times they only do just a small amount of damage, but they can sometimes paralyze your character causing him to fall down for about 20 seconds and twitch on the ground helpless. And in most cases, this is going to be just before a wyvern of some sort decides to charges at you turning you into a hunter pancake. It's less dangerous but still just as annoying, when you get paralyzed while carving or trying to harvest items as well.
Throughout Wild ARMs 3, you encounter a variety of owl monsters. The ones earlier on usually do their worst by simply being airborne enemies, meaning that it's slightly tougher to hit them overall, inflicting Disease on your party, and stealing an item that usually isn't too hard to replace like a Heal Berry, but the ones later on do all of the above, but when they steal, they steal and run, meaning if they successfully steal that precious Ambrosia or 2 you've been saving for an abnormality whore, then they're gone for good.
In the Geneforge series it was the Vlish, flying tentacled creatures with ranged attacks that had effects that made every combat longer; either stun, terror, poison/acid, some of them could even heal their allies. They ran to call on the assistance of others, came back to swarm in packs, then ran away when taking too much damage. And throughout the series, many entire zones were dedicated to these damn things.
The fifth and final entry of the series dialed the vlish annoyance up even further with podlings. Weird little headless torsos on legs, they used weak but large area of effect attacks that inflicted a variety of Standard Status Effects. They did little enough damage that they usually weren't that hard, but whether the player won or lost, the battle was sure to be soul-shreddingly tedious, and the few battles that included podlings and enemies that did real damage tended to be frustratingly lethal. And because Spiderweb games avert Money Spider, the player rarely even got decent treasure for beating them. For extra special bonus fun, there was a zone called "podling crossroads." Guess what lived there.
In the Avernum series it was the Imps, who were not nearly as bad, less frequent, and the rewards were better. They also assembled in packs, used ranged mental effects such as stun, charm, fear, and ran away at low health.
The Witcher series has drowners (and their improved counterparts drowned dead and mucknixers). Unlike most monsters they're more of a nuisance than a threat, but can take time to kill due to their speed and tendency to attack in groups while dodging sword swings. They're one of the most common species of monster and are found anywhere there's water. Beaches, oceans, rivers, swamps, bogs, sewers; go anywhere near one of these and it's almost guaranteed you'll get swarmed by drowners. And when you go diving in the third game they can only be fought underwater using a crossbow, which does little damage and you're better off just trying to outswim them.
There is a talisman in the first game that makes them flee when equipped, but it is a Power Up Let Down and makes them WORSE. What makes them so annoying is that during combat, you can't interact with items (such as opening a chest or door). The item keeps the drowners at a distance that STILL keeps them in combat, and keeps them too far away to instantly kill in group style. And to make matters worse, in the third game you can acquire the formula for a potion using their pheromones that's supposed to keep them from attacking you. To many players' dismay, it turns out that it only works in one specific quest and it's useless afterwards.
Wolf packs can be a nuisance as well. They're found everywhere, attack from every direction, and while you're swinging at one it can run off while another attacks you from behind. Leshens and werewolves in the third game can summon wolves to aid them in battle, making fights with those monsters even more time-consuming.
Baldur's Gate has plenty of weak enemies for the benefit of the new player, but Kobolds are its best example of Bats. Encountered in packs almost everywhere, they're incredibly weak and give insignificant xp for killing, but still pack a sting as archers. After you kill them, you'll find that they drop only short swords, shortbows, and arrows, which are all but valueless.
Baldur's Gate II has its entire first dungeon filled with Mephits, Fire Mephits, Air Mephits, and Mephit portals. Goddamned Bats indeed.
Any creature with level draining abilities becomes this, easily. Most of them aren't really *dangerous* damage- and health-wise, but they'll drain your characters to dry husks in minutes and take away your high-level spells. Most players quickly learn what things makes you immune to level draining, and uses scouts equipped with said immunity when vampires are afoot.
At higher levels, Baldur's Gate II hands you two "all bats must die" spells, Cloudkill and Death Spell, both of whom insta-kill low-level pests and are very good at cleaning out entire rooms full of Goddamned Bats before your fighter has managed to close with them. An otherwise Demonic Spider called the Umber Hulk is also extremely weak to these spells.
Any enemy with an activable shield in Zoids Legacy. You usually have to waste at least two party members' attacks to bring the shield down, plus the user can still attack through it. Fortunately, when these become common, the party should be strong enough that they do not become Demonic Spiders.
The .HACK//series of games has a few. The standout performers are Killer Bees and Mimics, both of whom are difficult to kill and spam negative status effects. In the words of Black Rose, "Confusion and Charmed are the worst!"
Let's not forget those resurrecting statues. If you faced two of them at a time (which you often did in the later levels/games) they would continuously keep reviving each other, forcing you to Data Drain one of them.
Any enemy with a "Evading" attack in Sonic Chronicles and high attack. And any enemy that you need to have "Piercing" to do damage to. And enemies with a high Defense. In short, around 30% of the enemies in the game probably qualify for this status.
Grand Chase features Harpies: starting from Kerrie Beach towards Marsh of Oblivion, your life will be made miserable by these flying creatures, even more so in Champion Mode, when they're buffed with all sorts of goodies. And then they make a grand comeback in the Battle for Bermesiah as Violent Harpies. What fun.
The Slimes in Odin Sphere's Titania level. They don't take a lot of damage from normal or Psypher attack. But the worst part is that if you don't get away, they will cling to you, and stick you in place, dealing their damage, while opening you up to attacks from Wizards. Generally a Napalm will kill them, but good luck if you don't have the stuff to make one, or had wanted to save them for a boss.
Dokunemons in Digimon World 2. Those guys are weaklings who'll probably die in a single hit. However, they can use their signature move to become untargetable for a few turns again and again and again and again... That being said, you can get that move if you manage to recruit or fuse one.
Bone Buzzards from the Nobilia/Crustacea region in Secret of Evermore. They do practically no damage, swarm in from EVERYWHERE, and can only be hit if they're really close to the ground. The buzzards, however, can hit you no matter how high they are. They also have no delay behind their attacks, and can hit you again immediately. When one makes contact, your attacks are immediately canceled even if it would have hit and killed the damn thing. Your only choices to deal with them are either to dance around for god knows how long, or waste spell ingredients (which is always pure overkill).
Bat type enemies in Star Ocean: The Last Hope quickly become this. They attack in packs, can fly (making them a hard target) and often cause the Fog Status Effect (the character can no longer aim or automatically find the enemy). Worse than them are the Bees, who attack in larger packs, move faster, are the second to third monster encountered, have an amazing ability to target the physically weakest character (usually Symbologists) and are the most responsible for many broken bonus boards. For those who want to know what a bonus board is, it's very useful, very hard to build up and is reduced either totally or by half with a single critical hit on the leader. No more 140% EXP bonus for you!
Thankfully rare in Vampire: The Masquerade – Bloodlines, but when you go to fight the plaguebearer Bishop Vick, he has zombie minions. They take several attacks to kill, they are literally endless, and while they're not fast, they can latch onto you and render you completely unable to do anything else until you throw them off. The best way to deal with them is to run past them, knocking them out of the way with a blunt weapon such as the baseball bat or truncheon, get to Bishop Vick's room, and defeat him as fast as you can. The implication is that without him around to feed them, they die of the plague and starvation.
There's a perfect example in the first boss (arguably a mini-boss) in Ys Origin. One of it's attacks is turning into a swarm of Goddamned Bats and chasing you around/moving in your path to swarm and bite you. It's made worse when the monster splits into two and has one of them swarm you as said Goddamned Bats, while the other tries to attack you with magic or some such. The bats they turn into are even an enemy you can find shortly after the battle!
Breath of Fire II gives us the Beak. When you first encounter it it's as the first boss of the game when your character is solo and pitifully weak, but at roughly level 30 it starts showing up in the hunting areas— and it's still just as strong. Curb-Stomp Battle ensues. So why is it on this page? Because every time you encounter one in the hunting area, you get kicked back onto the world map. No big deal, just avoid... wait, is it chasing after you? At speeds much faster than you can manage? Yes, these things are Too Dumb to Live and will actively commit suicide by your party, preventing you from getting any of the drops you actually went in there for. If you try going after it using the hunting area mechanics, it proves nigh-impossible to kill this way. And if by some miracle you do manage to do this? Another one shows up. And it drops absolutely nothing. Once these things start showing up in your hunting areas, you'll probably never use a hunting area again.
To a lesser extent, any enemy that has stat-modifying spells like Def-Up. Such enemies almost always come in groups of two or greater, and all of these spells share an animation that takes 8-9 seconds to go through, which is much longer than most spells in the game. Since the enemy AI prioritizes such magic above everything else, they will spam it with impunity in the first few rounds of the fight, meaning a single round of combat can take almost a whole minute to finish.
Any variant of the Shadowcats in Labyrinth of Touhou. When you first face them, they will be faster than anyone in your party, even Chen. They also know Shadowstep, which hits your entire active party and, more often than not, paralyzes at least half of them. This leads to trouble if you have to fight two at once and you can't get Meiling, the only person who can cure paralysis at this point, out on the field.
Later in the story, the bats are replaced for at least one day by rats instead. This is actually plot-relevant (unlike the bats, which are just annoying) because it's a sign that something's wrong in the sewers.
The Blighttown Giant Mosquitos are always flying and thus hard to hit in melee, so weak as to be a waste of a spell, an attack that's not too troublesome on its own but can shatter your guard if there's more than one(and there usually is) that also help cause poison, to help finish the infliction if the poisonous waters of the swamp didn't already. And to top it all off, they hardly give any souls and respawn several times in a row.
Dogs in any Souls game are universally hated by the players. While they go down easily, generally in one or two hits from just about any weapon, they are very, very fast and their bites and claws do a lot of damage. Fighting a group of them at once is never fun, and one of the most hated That One Boss'es in the series is infamous in part because he has two dogs accompanying him. Taken up to eleven in the Spiritual SuccessorBloodborne, where dogs are just as nasty but there is no blocking mechanic, meaning you now have to out-speed and out-dodge them, which is tricky even with the more agile Bloodborne characters.
The Dark Spire. One of the most common early enemies? Literal Goddamned Bats. Most of the other common enemies early on are also Goddamned Bats in the figurative sense. Then you run into the bats that look almost exactly like the first kind but are actually Demonic Spiders...
The TerraTerrors in Terra Tower, the penultimate dungeon in Chrono Cross. They're everywhere, actively chase you, and have a tendency to spawn Cytoplasms in battle, which endlessly use weak techs. At this point, none of the enemies can damage you enough to be a real threat, so getting into fights is tedious and pointless. It doesn't help that the game is suffering from serious Ending Fatigue at this point; these fights only serve to drag out an already-tiresome Marathon Level.
Prehysterics in Gaea's Navel. They're no real threat, but they always lead off with an attack that confuses a party member (thus dragging the fight out) and has an unnecessarily long and annoying casting animation where they dance around, hooting and banging a drum. For good measure, they sometimes appear in groups of three, potentially confusing your entire party.
As you get better gear, previously challenging monsters go from threats to annoyances, but they still cause knockback even if they cause no appreciable damage. Even after you have a full set of Molten armor you can still be killed by a lowly slime slapping you into a pit of lava. That is, unless you have a Cobalt Shield or its upgrades to prevent knockback, in which case they go from a minor threat to practically nothing.
Any enemy if you're trying to manage your items or craft something, especially worms.
Want to get your hands on some Meteorite? You'd better have the patience of a saint, since Meteor Heads will be constantly harassing you as you try to pick it up. The Meteor Heads that spawn around a fallen meteorite are a good preview of what to expect while mining Hellstone. They die in one or two hits, but they spawn constantly, swarm the player, set them on fire, and they don't stop coming until you've mined enough meteorite ore. You'll start being really tempted to just blow everything up with a few sticky bombs and call it a day.
The game has literal bats, and though they aren't that dangerous as long as you have good armor (save the Giant Bats in Hardmode that confuse you), they spawn constantly. On top of that, their hit-box is a single block, so ranged weapons have a hell of a time scoring a hit. This is especially true in the Underworld, where up to five at a time may be harassing you, and down there they do enough damage that even Molten armor isn't enough to render them harmless. Considering you have to spend a not-insignificant amount of time down there trying to flatten the place out for the Wall of Flesh, this can get old fast. In Expert, they're even more annoying—getting hit by them can give you the Feral Bite debuff, which increases your damage, lowers your regeneration... and also periodically gives random smaller debuffs on you, such as Confusion. It takes an entire minute to wear off.
Spellcasters — monsters that teleport at will to anywhere nearby, and can shoot their projectiles through the wall and love teleporting to places you can't reach without tons of digging. Thankfully, most projectiles can be swung at with a melee weapon or tool to get rid of before it hits you, but still. Trying to mine Hellstone in The Underworld is especially annoying when Fire Imps keep pestering you every 5 seconds, especially when there's lava everywhere. Once you actually close the distance with one of these bastards, they die quickly and easily and tend to just stand there while you smack it to death. The only problem is that another one will spawn in just a few seconds...
Harpies. Flying enemies that appear at high altitudes and use a high damage projectile feather attack that shoots out multiple projectiles. Doubly so if you're trying to fish up there.
Archers are annoying as heck. The hit you from a long way away, do hefty damage, and the Skeleton variant can set you on fire.
Lava Slimes on Expert Mode. They drop a block of lava when killed, which can deal tremendous damage just by itself. But that isn't the worst of it. If you're trying to build an arena for the Wall of Flesh, the lava will break wood platforms and take out campfires. Even if you're using ash blocks, you still need to poke holes to drain it out.
The various Eaters in the Corruption, because they just never stop coming and spawn really quickly. The Crimson is better because its variant spawns less frequently.
Gastropods shoot pink lasers from off screen with ridiculous speed and accuracy and deal massive damage for a common mob. Plus, they tend to gang up on you.
Chaos Elementals run like crazy, charging you and teleporting all over the place.
For a new player or someone setting out with a new character, Flying Fish are difficult to deal with due to their numbers.
The Lunar Event is chock-full of them. Most of them also have a great amount of health, requiring the strongest weapons to bring down.
Nebula Pillar: Brain Sucklers appear in huge numbers and fly straight at you, even through walls. When they hit you, they latch onto your head and greatly obscure your vision and deal a good amount of continuous damage. Nebula Floaters fly around, fire painful projectiles, have a good amount of health and teleport when hit.
Stardust Pillar: Star Cells are Asteroids Monsters that, when killed, produce four smaller cells which can each grow into a new Star Cell, creating a potentially endless swarm if you can't track down the cells or don't have a weapon that deals damage over time like fire. (However, since progression through the event depends on the number of enemies you kill, Star Cells can be used to block any other spawns and make progression very easy, provided you can avoid them.) Milkyway Weavers can pass through walls and can appear in numbers.
Solar Pillar: Corites, Srollers, and Selenians. All three appear in numbers, the first performs a homing fireball tackle that deals good damage and passes through walls, the second and third perform a rolling attack that doesn't pass through walls or home in, but hurts even more, and the Selenian's rolling attack deflects projectilesback to you. And if one attempts to avoid them by flying... the Crawltipedes are there to make sure youdon't.
Vortex Pillar: Storm Divers can fly around, and fire a very painful shotgun attack that can be tricky to avoid. Thankfully it's easy to hide from them behind terrain and use a penetrating weapon to beat them. Alien Queens spit a projectile that causes the Distorted status, causing the player to float about uncontrollably. Finally, Alien Hornets can sometimes be spawned right on top of you by the pillar for cheap Collision Damage.
Tyrant Aciel in Shin Megami Tensei III: Nocturne's final dungeon. They resist all magic, cast a spell to become effectively immune to Physical if they ever get a turn, have a large amount of HP and as a Tyrant, you can negotiate with them, but they give out half the EXP of other, easier to kill, foes in the same place. They aren't strong enough to actually kill anything that is at full health though.
Infinite Undiscovery has green baby giant spiders called Larva. They are either in packs or jump off of the corpse of its parent that you just killed. They can get incredibly annoying because of their ranged attack (and in this game, even that attack can stagger you and interrupt your attack) and the fact that they can easily and quickly swarm you. They are only really bad in the part of the game where the hero is briefly separated from his allies.
Dragon's Dogma has harpies and their advanced form, succubi. Individually, they can be taken down with one or two well placed attacks, but good luck getting them off. These creatures fly, sometimes erratically, cast sleep, can knock you down, stun, grapple and do flat out dive bomb attacks. To make matters more interesting, they tend to spawn in groups and have no qualms about harassing you in the middle of a miniboss or full boss fight.
Parasite Eve has especially annoying actual (mutant) bats fluttering around in the Chinatown sewers, capable of reducing your attack range to zero with a sonic attack, and mutant crows that constantly move and peck away at your HP, dropping only piles of junk when killed. That junk can be put towards earning extremely powerful weapons, however. You'll just have to kill 300 crows to get enough.
Pararoids and Metaroids in Mega Man X: Command Mission. They're very fast and hard to hit, and while all they use is their regular attack and one ability, that ability is Parasite, which sacrifices themselves to raise another enemy's stats to the max. There's always at least two of them in a given encounter with them. Metaroids are even worse because they don't even have the regular attack; as soon as they get a turn, they use Parasite. Which they will most of the time because they're the fastest enemy in the game. Meaning that you have to be able to kill them before they can take a single turn. If you do manage to kill them, you'll find that they drop...absolutely nothing.
Every enemy with a ranged attack. Since they can't be blocked, those can chip away at you while you deal with their tougher, melee buds. Of course, nothing stops you from simply running to them.
Nymphs. Very hard to hit, moving constantly all over the place and peppering you with projectiles. They can also summon additional enemies that, while not really powerful, still annoy you and require to be killed... and she can always summon more.
Tougher undead block most attacks flawlessly, have a few deceptively long-ranged unblockable swings and take a while to die. The toughest version can also summon additional undead to aid it.
Though early on they're Demonic Spiders, Minions in both forms quickly degrade to it, for better or worse. They're very adept at blocking and interrupting your combos and are durable enough for even an end-game hero to take a while to finish off with magic.
Trolls are normally fought mano-a-mano, but in these few occasions they have a back-up (of, conveniently, Minions), they become this. Their attacks can't be blocked and will knock you down, leaving you vulnerable to getting swarmed. They also take a while to kill.
Etrian Odyssey IV has Goddamned Vampire Bats in the last leg of the final dungeon-they can't do much in the way of damage, but they can drain HP, have enough health that they can CONSTANTLY call in another bat just before you can finish the last one off, and to top it all off can team up to perform a skill that drains a massive chunk of TP from everyone in your party when there's three of them in battle. And for maximum irritation, they can also be accompanied by a beetle enemy that can sponge all attacks directed at them! At least both the beetles and bats are weak to electricity so a multi-hitting electric move will get rid of the beetle in a jiffy, but that can still give the bats a turn to call in reinforcements or do their TP-draining move.
The tiny two-legged enemies in Pandora's Tower. They're very fast, tend to appear in groups of three or more, and their attacks have little "tell," making them difficult to dodge. One at a time, they're stupidly easy to deal with (as any binding with the Oraclos Chain renders them completely immobile), but in groups, they can quickly accumulate damage. And if they're Purple Misted, they can quickly become Demonic Spiders for their hard-to-dodge attacks which suddenly knock off a fourth of your health per hit.
Bats, Birds and Wasps are all damn near impossible to hit in any game, and were just listed in increasing order of annoyance. Bats are the least annoying, even though some can drain your health. Birds can buff their team's evade (already high if there are more around, which there usually are) or stun you, depending on the species. Wasps... these nasties can do anything either of the above can, not to mention poison or otherwise debuff you, and cast magic on top of that, and they have a healer species to boot, and any kind can summon more at will.
Clays count too. They all either strongly resist, ignore or even absorb many elements, all different, and only have a couple common weaknesses, Earth and Bomb. Only Lance (or Anna in 4) can sweep in either of those elements (and only Anna can sweep in wind, the only thing they're all at least neutral against), and they often appear alongside enemies that resist, ignore or absorb Earth too. Each of them can also buff its entire team in a different way, and the two new kinds introduced in 4 have nasty special moves.
In the same dungeon in 4 that introduces the two new Clays (and adds in plenty of Wasps for good measure) also introduces four unique kinds of flower. Florns dish out strong poison sweepers and self-destruct at low health. Heasies heal or buff their party and cast Earth magic. Stunflowers can stun you. And Froses can wet you and then strike with ice moves powered up by the wetness (or just pass the popcorn while their Stunflower friends' electric attacks get the same boost).
The last dungeon adds even more: Ghosts automatically revive in the first five turns; not at full health, but they still have high evasiveness.
Compared to Mage Birds and Dogs, though, ghosts are the least of your concerns. They often come in large groups, and the birds share all the strongest traits of all other birds and can stun you for two turns, greatly buff their allies' (in the case of each other, already sky-high) accuracy and evasiveness, and worst of all drain your MP, while the dogs can syphon you, almost completely heal their entire party, and buff their allies' attack and magic even higher.
In Citizens of Earth, the Cone Crab enemies don't pose too much of a threat to your party, but they are incredibly annoying to fight due to their ability to hide in their shells, which makes them completely invincible for that turn and the next one, in addition to restoring their HP during the second turn of hiding. And worse, they're very common in Capital City and surrounding areas.
Ellin and Ellen in Dark Grave. Not too dangerous, except Ellen has a small chance to inflict death. However, they start the battle with a shield that protects them from any kind of damage for two turns. Hopefully, you have units that has Piercing attacks or attacks that reduce buff duration. Else, you need to actually wait 2 turns to kill them. Getting three stars on Dark Grave levels are annoyingly hard due to them.
Sarah in Revenger's Hell. She has no damaging attacks, but she just loves to spam the skill that reduces your damage by half. Not to mention she herself has a passive that halves damage taken, meaning you only deal 25% damage to her. Try using debuffs on her? Too bad, she also has a skill that removes debuffs. And she often appear in flocks, so even if you Stun or Paralyze a Sarah, the other Sarahs will quickly get her back up again.
Daisy in Moonlit Isle, who has a passive ability that reduces damage from all-targetting skill to her allies, which you generally want to use to quickly wipe out enemies. In addition, she often appears together with the more dangerous Rei, who herself also takes less damage from all-target skills. Even more annoying, she and Rei often hangs out at the back position, with the tanky Jin defending them in the front.