Wild ARMs XF: Very common enemy starting from the beginning in non-storyline battles is the "harpy." (It's a bat.) Has special attacks that can steal your items, confuse you, make you unable to heal, diseased, miserable, or insta-kill. Did I mention that it's probably faster than most of your characters? Did I mention that it's also more agile, and will dodge most of your attacks?
Interestingly enough, Harpies are part of a glitch that can be used to get massive amounts of money very quickly by duplicating an item from 1 to 255. The problem is, they need to STEAL that one item you want to duplicate. Which you then sell to the store for massive money.
Demonic Spiders in Fire Emblem games are easily Wyvern/Draco/Dragon Knights and Mamkutes/Manaketes/Dragons/Dragon Tribe Laguz... The former especially if they appear in the games' earlier chapters where you still don't have powerful units/weapons just yet, and they are a 'lot' more common than the latter. This doesn't mean the latter is any less nasty though...
Wyvern/Draco/Dragon Knights are pretty nasty opponents, they have very high strength and defense which makes them difficult for your melee units to kill. They are also flying units so they can just zoom in at any opportunity to harass you once a unit is in their range. And despite being flying units, their defense is often so high that they can laugh at their apparent weakness to arrows, unlike their Pegasus Knight counterparts. And they love to pick off weaker units and healers thanks to their higher movement range and the fact that in most games they wield lances so getting ambushed by one with a Javelin is not uncommon. And in the non-GBA installments, they will (or at least attempt to if they have movement spaces left) fly away from their target to terrain non-traversable by ground units after attacking, denying you the opportunity to finish them off when your turn comes. And they often spawn in said terrain so they will almost always get the first strike. The third thru fifth games were by far their worst. Magic, especially Wind (or Thunder in FE10) Magic, and Wyrmslayer swords are about the only effective tools to dispose of them.
Now for Mamkutes/Manaketes/Dragons/Dragon Tribe Laguz, These guys only appear in a couple of games, most notably Marth's and Roy's, and usually in the last quarter of the game. But these guy are often the hardest hitting enemies in the games outside bosses, since many of them have attacks that ignore both defense AND resistance. Some of them are even immune to magic, killing the mage strategy and forcing to use a melee character and/or the mentioned Wyrmslayer, and to make it worse, even with that sword they can still take at least two good hits to kill unless the unit crits or is very overleveled thanks to having even higher stats than the Wyvern Knights. The only upside to them is the fact that their hit rate is atrocious, but without dragonslaying weapons or strong magic, trying to take those monsters down is very risky and tedious.
The Wyverns in Mystery of the Emblem and its remake deserve a special mention though. Remember all those things said about high-movement Wyvern Riders above? Yeah, immagine that on a defence-ignoring, fire-breathing dragon. It's just as bad as it sounds. Not to mention the first time you encounter them is in a desert map, massively hindering your own movement while they can just fly around picking characters off at their leisure. And as if that wasn't enough, on Lunatic Mode their breath has increased range. Yeah.
The literal spiders (okay, Bael) in Fire Emblem: The Sacred Stones, provided they bear the Sharp Claw weapon. Like Wyvern Knights, things are nasty early game where most units have barely 20HP, they are practically 1HKO'd since the Sharp Claw has 14 attack before the damage calcuation is factored. They get easier later on, thankfully, and they also are balanced out by a low accuracy.
The Gorgons also qualify. Fairly strong magic attacks, which could be murder on units with low resistance, but the worst is Stone, which completely immobilizes a character and makes the enemy unable to miss and increases the petrified unit's chance to be criticaled by a whooping + 30%. This is a game where there's no reviving,, thus if the character dies, you either have to restart the mission or deal with never being able to use him/her again.
The Draco Zombies from the same game are borderline boss in mook clothing. Incredibly tough, take ages to kill, and their attacks are nigh-undodgeable and ignore your units' defense (because of this, their attack will always do exactly 32 damage - more than half the maximum possible HP for any of your characters). And the final floor of the lagdou ruins is filled with them. The only saving grace is their weakness against bishops and arrows.
The ROM hackFE Girls has Whispers. They have no map sprite, an absurd movement range, make no sound when moving, and one of their inherent abilities is Critical+100. Meaning that whenever they attack you, unless you've been abusing Supports, it will be a critical. They also have Bane, which has a (Crit/2) chance of making the attack HP to One. They also appear predominantly in maps with Fog of War, making it that much harder to see them.
FE4's got: Any enemy with Thunderstorm (Bolting), Blizzard, or Meteor. (Infinite uses, long range attack, will probably hit and do tons of damage.) These show up in other FEs, but are no where near as annoying (And can usually be soaked through. The ones in FE4 have infinite uses, it's five charges in any other game.) It gets even worse in FE5, where Blizzard inflicts Sleep. Which in turn results in your units being instantly captured if an enemy so much as touches them.
Or worse, Dark or Lopto Mages and Hell (Eclipse) it reduces the Target's HP to One — and because at least one damage will always be dealt from any successful hit in FE4, any hit after taking a Hell means you're dead.
On the other hand, you can unleash your own Demonic Spider on the enemy: This one lets you have the dread "Berserk" Staff... you know the one that'd make Colm turn around and kill Neimi. In this title, status-effect staves automatically hit if the user's MAG is higher than the target's RES and very few non-magical units have significant RES, making these staves monstrously powerful for either side.
3-hit combo from X-Com: Cyberdisks, Psi-using aliens, and Chryssalids. Cyberdisks can fly and come with an accurate plasma cannon capable of rendering your units dead in a hurry, along with the fact that they react violentlyto dying; the one advantage you do have against them is that they're much larger than a normal enemy and thus easier to hit, which is just as well because the only starter weapon that can reliably One-Hit Kill them is the rocket launcher. Oh, yeah; there's also a 50% chance that they'll show up in the first month, along with a psi-using alien. And those guys are fun, too. Once one of their allies spots one of your units, the psi-user amongst them gains the ability to go through your entire team for the unit with the lowest psi-defense (that you can't even see without the proper upgrade) and promptly makes that unit their bitch. Enjoy having that unit either run around randomly without their weapon, fire all over the place, or get Mind Controlled and take out 2 or 3 of their former teammates. The only way this crap gets more broken is when you get it. And then there are Chryssalids. Oh dear God there are Chryssalids. These things have one attack and one attack only: An infectious bite that instantly converts whatever was hit into a zombie (And kills tanks in a hurry). Said zombie becomes a new Chryssalid upon taking sufficient non-fire damage, which can then convert more units, and so on. And there is absolutely no defense against this bite: Your Super Soldier of a Commander falls just as easily as your Rookies. On top of this, they possess enough Time Units to charge at a unit from beyond visual range and transform it into a drooling zombie before the player is even sure it's there. Upon finding out that these things are involved in the mission, players are known to take drastic measures such as carrying a primed grenade at all times, killing civilians in the area to prevent them from being infected, and blowing up the body of every single Chryssalid that goes down, just to be sure.
Then again, Chryssalids will be rendered rather harmless once the player acquires the Flying Suit: Their horrible attack won't reach you on air.
In case this isn't enough, X-Com: Terror From the Deep replaces Chryssalids with Tentaculats, floating brain monsters that do the exact same thing. Except these sons of bitches also fly, so all those clever people that thought Flying Suits saved them from the Chryssalid plague in the first game will find that their floating Mag-Ion Armors do absolutely nothing in the second.
The game also replaces the Cyberdisks with Bio-Drones, which have the same traits as the Disks only now they're smaller, harder to hit, and it's near impossible to tell where they're facing making it real hard to sneak up behind them. Trying to attack them from the front is practically suicide since they have a high reaction stat and near-perfect accuracy.
And X-Com: Apocalypse continues the fine tradition with Brainsuckers, which don't transform targets into more brainsuckers, nor do they have a 100% success rate, but make up for it by the fact that some enemies can fire them at you. Y'know, in case the little buggers can't reach you on their own, 30 foot vertical jumping range. And while the Chryssalids and Tentaculats (Along with their zombie hosts) could only do melee harm, Brainsucked soldiers can and will shoot at their former comrades. This gives them the dubious honor of being a horrible combination of the mind control the first two games' psychics had and the Body Horror of their predecessors.
And let us not forget the Poppers. These are ridiculously fast suicide bombers which not only explode once they're near your troops, but also explode from almost every type of weapon you have, and their explosions are huge and very damaging. Apocalypse players have learned to fear the little pitter-patter sounds they make.
UFO:Aftermath has the aptly titled Deathbellows. It shoots swarms of bees, that scatter on reaching the target, Evaporates health so quickly you barely get time to pause the game, swarms are able to slowly chase any squad member able to dodge the first attack, have no limit in the number of swarms active, are fired rapidly and to cap it off the range of the Deathbellows is entirely line of sight. If it starts on land that's a an inch higher in elevation on a sparse map, then kiss your squad goodbye as Total Party Kill seems to be its only purpose. Two of these bastards present on a map guarantees the loss of any team member with less than maximum speed. Three... Well lets just say you'll be thankful for the autosave. Its possible to kill, but only if you've got the most blessed terrain setup and a maxed out sniper and then only just.
Reticulans have a superior rapid firing rocket launcher, compared to humanity's RPGs and LAWs. Not to mention they weigh less then a feather and some plot missions occasionally spawn several Greys with these together. Oh and don't forget the near instant-kill damage for your Powered Armor troops, the rest might's well be naked for all the good armor does.
XCOM: Enemy Unknown brings back the Cyberdisks (now with the ability to chuck grenades!) and Chryssalids, though it arguably nerfs psi-using aliens to just below the Demonic Spider threshold as they now rely on line of sight. However, it adds Thin Men, who have the ability to spit a poison cloud with 100% accuracy, a high accuracy with their light plasma rifles, and burst into a poison cloud when killed.
Bonus points for these Chryssalids actually looking like a cross between a spider and a hydralisk and being able to leap on and off the top of buildings. However, they have been nerfed to the point where their attack doesn't mean instant death. They can only infect those they kill, and you can further protect your soldiers by equipping them with the Chryssalids' own carapace. The Sectopods, like in the original game, are still a major pain in the ass (especially since they can perform two attacks per turn with their Wave Motion Gun and still use their reaction plasma turret or launch a missile barrage), although a Heavy with certain skills and a rocket (or guided fusion) launcher can make short work of them in later stages.
Anti-Tanks in Advance Wars: Days of Ruin qualifies to a slight extent. If there are only land units, what makes it the epitome of Crippling Overspecialization (killing tanks efficiently, short range, weak against an infantry swarm) makes just one of them a pain to deal with properly. This can apply somewhat to indirects, but the Anti-Tank is the case worth mentioning, considering that it slows you down way worse. At least you get Anti-Tanks first though.
Don't forget that they can just as easily blast Battle Copters out of the sky, surprising at first, considering that the only ground unit that could handle them before were Anti-Airs and Missiles.
Ironic that the game's Demonic Spider is vulnerable to the games' Goddamned Bats...
In Final Fantasy Tactics Advance, some enemies (the Antlions) learn a technique that instantly kills any of your party members providing their level number ends in five or zero. So if you go into a battle with them with a group leveled at those numbers, there's pretty much no way to win.
These spells have been a staple of the Final Fantasy series since time immemorial. However, it should be noted that in FFTA, your maximum level is 50, which makes leveling to avoid the requirement impossible. Other Final Fantasy games have had the level cap at 100 (same situation applies), or 99, in which case your nemesis is the spell that damages characters at levels with multiples of 3.
Fairies are surprisingly deadly, especially with their "Level ? Holy" (holy damage on units with the same last digit of their level) and Angel's Whisper (heals and gives auto-life to one of their units), making them easily able to complement other monsters.
Malboros of Final Fantasy fame are pretty much guaranteed to be this in every game they show up in, but the worst case is the Cassie variety in Final Fantasy Tactics A2. They can heal each other and use a number of buffs, put your characters to sleep or Charm up to 4 of them at once. At a 100% sucess rate. At the same time, they wear Ribbons, making them completely immune to Debuffs. And did I mention none of these abilities cost any mana?
Cockatris in Ogre Battle, these creates canwill turn your characters to stone for the rest of the fight. This wouldn't be so bad unless A.Ogre battle expected you to "win" most battles (rather than force retreat via dealing more damage, then following it up once or twice THEN dealing enough damage to kill, meaning your chance of winning is lower because you don't have that character dealing damage) and B.said monsters didn't have godly dodge and accuracy.
And in OgreBattle 64: Person of Lordly Caliber, the petrification status effect lasts even after the battle ends. The unlucky unit afflicted with it stops gaining EXP and is treated as dead. On the plus side, the computer treats a petrified opponent as dead and you can easily wipe out enemy units that way (though you won't get EXP for them.)
The frigging Winged enemies early in Disgaea. They've got the best mobility in the game when you first encounter them, and they can spam an area attack that inflicts Universal Poison before you probably have the spell to cure it (and certainly before you can cure multiple people at once). Ironically, running into higher-level and more powerful versions later becomes easier than their first appearances; the A.I. Roulette will cause them to use attacks they learn later on more frequently, which are much easier to deal with. Plus, by then you probably have several characters that can cure poison (presuming you don't just stock up on the cures at the store).
The winged return in force in Disgaea 4: A Promise Unforgotten. Their base evilty (as in the one they always have) halves the accuracy of all attacks made from an adjacent space, making them a pain for most melee classes, especially axe users. The game also loves throwing them in with "No Ranged" Geo Effect panels, making them infinitely more aggravating. The game also has Ninjas. Now, Ninjas have been in every Disgaea game to date, but never have they had the ability that, like the Winged above, halves accuracy of certain attacks (from the front this time). Normally, these enemies only appear in small sqauds. However, Chapter 4 breaks this rule by having just about nothing BUT Ninjas (labeled Office Workers), in almost painfully small maps where being anywhere but in the frontal cone is either painful or impossible. And then a Winged magichanges with a Ninja...
In Medieval II: Total War, the Catholic Church will send Inquisitors into your territories if you get excommunicated, or they'll just wander into your lands on their own, and start setting agents and family members on fire. The only way to stop the Inquisitors is to assassinate them, which is spotty at best.
The whole damn Catholic Church is a Demonic Spider (no pun). The Papal favor rates are entirely random to start with, and are almost entirely random throughout the game, as you can LOSE favor for not building enough churches, or even while building plenty of churches. It can get to a point where you can be struggling merely to exist, and the Pope will FORCE you to devote your glorious faction leader to a pointless Crusade of Buttfuck, Nowhere or else be excommunicated. Even worse is when another Catholic faction decides it wants your land, attacks you, you attack it back, manage to retake your land, then while you're besieging your old capital, the Pope tells you to STOP FIGHTING WITH SAID FACTION and that if you attack them again within 10 turns, you will be EXCOMMUNICATED! Even just taking back your old capital will get you punished, and in the end THEY will get off with HUGE Papal favor!
Is it a surprise that many veteran players make it their priority to take Rome ASAP? Given a free hand, the Pope goes militant and starts taking over half of damn Europe.
Now THAT'S what I call a good history simulation!
Also, the goddamned Mongols. In every game they appear in. They show up out of nowhere with a huge army (in Medieval 2, it's seven fully stacked armies) made up of troops with incredibly high experience and morale. Almost all of their troops are heavy calvary and horse archers, making them unbeatable in open battle. Even if you manage to bait them to attack your cities (generally considered the best strategy, since it negates their movement advantage and basically makes them run into your spearmen), they're insanely tough to beat due to their unwillingness to retreat and pure numbers.
In naval combat, we have TIE Defenders, as usual. Any Rebel fighter, and therefore any Escort Carrier. The Death Star with a Death Star shield makes a battle unwinnable. Interdictors are particularly demonic when combined with the above.
Force-sensitives are the only ones that can level up. They are particularly demonic when they do. By the way, Vader starts at Jedi Master level, has a ridiculously high Combat stat, and Luke encountering Vader and not getting captured to learn of his and Leia's heritage qualifies as that game's That One Sidequest.
The Swarm, aka space bees. They will eat small groups of ships with ease if you have no point defences or strong rapid-fire weapons.
Von Neumann machines "consume" your ships quite quickly and can take a fair bit of punishment from small weapons. Their strong point defences make missiles useless except in giant Macross Missile Massacres, and missiles will probably be your most reliable heavy weapon early-game unless you rush something else at the cost of economic or development technologies. And that's just the little ones.
The Crows, aka Morrigi Durable Deathtraps. No problem for a Morrigi player or anyone who's researched the third-level Morrigi language, but anyone else is going to be either dragged into the planet's atmosphere, beset by asteroids, or just outright swarmed by drones.
Super Robot Wars Alpha has the Ghost X9. They have one of the highest dodge rates in the game and extremely good movement; without the Strike spirit command, they will fly around depleting your units' health until you manage to land a hit; units with lower armor will be hard-pressed to survive, and they can drain valuable energy from counterattacking Supers. They are less annoying in Alpha 3, with the exception of a stage that shall not be named.
Super Robot Wars Z2 has the DAMONs, Eldritch AbominationMooks with high dodge, armor, a very powerful and very low-energy barrier effect, and powerful attacks that drain both Will and Energy. Also, they turn up a lot. However, most of the attacks can break through barriers.
Any Festum successfully hitting an allied unit drops the pilot's Will and armor value. While these Standard Status Effects aren't such a big problem, the "demonic" part lies with all Festum units having an ability that grants them greater accuracy and evasion rates against non-Fafner units (and Heroman).
Individually, ELS units aren't strong, but they come in large numbers. Alongside dealing normal damage to HP, they also absorb EN, at a minimum of 10% per attack. UX treats the case that if any unit's EN is rendered to zero by ELS, it is automatically destroyed.
The first candidate is the V2 soldier, which first appears during an Escort Mission. V2 Soldiers are essentially invulnerable until the Supply Vehicle powering their shields is destroyed, and constantly pepper the player's squad with hails of highly damaging laser fire. However, with high-end equipment, they become less threatening, reducing them to Elite Mooks.
The second is the Ghost Tank, which appears in the post-game missions. They have ludicrously high HP and defense which can even shrug off a Lancer Elite or Mauler attacking from the rear, and they have souped-up versions of the V2's laser. In the very last non-DLC map of the game, the squad starts with three of them right from the start, each of them having over 5000 HP.
Warhammer 40,000: Chaos Gate has the Flamers. These demons are tougher than your average space marine, quite fast, can set your guys on fire (meaning they take damage and are effectively helpless until it goes out), cannot be set on fire themselves and, worst of all, they will simply set you on fire if you try attacking them in melee.
In first two incarnations of Heroes of Might and Magic both ranged units and flying units qualify: The former because of how most units is fairly slow(especially for Knight and Barbarian castles) which allows them to shred you from afar, the latter because of wonky movement system which allows them to jump to your ranged units and start massacring them. Third game nerfed their movement abilities a fair bit, although some faster units are still able to jump to you in one-two turns.
The Squeakers from Odium, thanks to their earthquake attack, which not only causes a lot of damage, but it works in a very large, cone-shaped radius, making it very hard to keep your men out of the range of their attacks. Very often, you'll find a Squeaker or two managing to suddenly nail your entire team in an attack you never noticed was possible.
Ghosts in Heroes of Might and Magic II. They're decently strong for a level 2 creature to begin with, but their special ability is killer. Anything killed by a stack of ghosts turns into a ghost. This means to fight them, you have to attack with only high enough level creatures to prevent the ghosts from killing too many things and replenishing their numbers. If you make the mistake of fighting ghosts with level one creatures, watch in horror as a stack of ten ghosts hits your stack and is suddenly a stack of 30 ghosts.