Metal Slime



A normal video game Mook is a monster that shows up regularly in large numbers, exhibits Suicidal Overconfidence, goes down in a sword strike or four, gives a little experience, and drops items that are either Vendor Trash or subject to the Antidote Effect. A Metal Slime is everything but normal: an uncommon monster with an uncommon reward that's uncommonly difficult to obtain from the monster before the battle ends.

To be a Metal Slime, an enemy must satisfy all three of the following criteria.
 * The enemy is an uncommon encounter, whether by means of rarely spawning, appearing only in out-of-the-way locations, or only appearing a fixed number of times.
 * The enemy has an unusual reward, such as bonus cash, bonus Experience Points, a higher chance for an item that Randomly Drops, or a unique (though not necessarily useful) item.
 * The enemy has a reasonably reliable way to end combat without the player being defeated or getting the reward, whether through the normal "flee" mechanic, a special ability, or a feature of the zone that it appears in. In games with Preexisting Encounters, this trait may manifest as the enemy avoiding the party in order to prevent the beginning of combat proper. As a result, it is difficult to obtain the reward from the enemy.

Most of the time, the enemy will give the reward for its defeat, making the challenge to kill it before it escapes. A variation is an enemy that is lucrative to steal from—in this case, the issue could be that the enemy gets itself killed before you have a chance to relieve it of its Infinity-1 Sword or Rare Candy.

Assuming it's not of the "strip it bare before it dies" variant, there are many ways a Metal Slime can be difficult to kill. If the enemy has a very high chance of escaping, it may be a feat to take action against it at all. It could also have high defenses, such as a 98% chance to dodge attacks, or defense power so high it reduces all your attacks to Scratch Damage. Alternately, some enemies can disable your party's attacks outright, leaving the player with no way to take effective actions against it. Depending on the combat system used, there may be methods of survival that are even more unusual, of course.

A Metal Slime is a specific variant of the Goddamned Bats—it is more annoying than deadly. Though most Metal Slimes lack significant offensive output, enemies are only excluded from this trope due to power level if they pose a significant threat to a party at the appropriate level for the zone.

Given the nature of the Metal Slime, it can be a major target for players working on an Elite Tweak or 100% Completion. If an item/cash-type Metal Slime ends up having an esoteric weakness that gives the player a decent chance of killing it (or if the rewards are just that good ), it can also become a Pinata Enemy.

This trope is named after the Metal Slime from the Dragon Quest series. Despite being most common in RPGs, a Metal Slime can appear in games of other genres. Not to be confused with a Metal Slug, although finding those is a fun reward in and of itself.

Action Adventure

 * The Mini Smiles in Killer7 fly at you at high speed. If they hit, they explode and temporarily blind you. Shooting red ones gives you a great deal of Thick Blood (which you use to power up), and shooting a yellow one heals you fully. The Camellia Smiles, likewise, run away as soon as they're shot and explode a few seconds later; killing them before they escape (which requires hitting their weak spot or a very fast character) rewards you with large amounts of Thick Blood.
 * The Takkuri in Majoras Mask, which drops a lot of money, but may decide to steal one of your items and run away with it, forcing you to wait until nighttime and buy it back from its owner. Yeah.
 * The game also crosses this with Die, Chair, Die! in the Keaton nests, where you can destroy a hexagon of seven shrubs, one green rupee for the first six and one red rupee for the last. The Metal Slime part comes in where they start running away as soon as you attack the first one, and are just barely too far apart to get in one sweep of the Spin Attack before you get the final sword upgrade, and bombing them requires very careful placement of the explosive.
 * Castlevania Aria of Sorrow had the Tsuchinoko, which would flee as soon as you entered the room about 90% of the time. If you didn't kill it fast enough, it would flee anyways. Compounding this is the soul it Randomly Drops which is, of course, needed for One Hundred Percent Completion. Making it better: Tsuchinoko Soul decreases the buying cost of items, including the ludicrously expensive accessory that makes Souls easier to get. So, um, you can grind money for the Soul Eater Ring to make it easier to get Tsuchinoko, or you can camp Tsuchinoko to make it easier to get the Soul Eater Ring.
 * And, of course, Tsuchinoko requires that you go back into a boss room, which you have no reason to do. And it doesn't always show up, either, potentially making this a Guide Dang It.
 * Don't forget the damn thing spits poison at you if you're not careful, reducing your attack power.
 * Let's not forget the Sky Fish. They move (and thus flee) so fast you need a power that STOPS time just to slow them down (stop them? Yeah right) to the point you can try killing them. Trying to time an attack perfectly so that you connect with one just when it's whizzing by or turning the area where it'd appear into a minefield wouldn't work either, since it can only be damaged with time slowed down.
 * Portrait of Ruin had an enemy called the Dodo. It appears on only one screen in the entire game, sometimes doesn't appear at all, has a decent amount of HP, and it runs away from you right when you enter. You have to kill it to complete a quest.
 * The Dodo originally showed up in Symphony of the Night as well.
 * Castlevania: Harmony of Dissonance had the Rare Ghost enemy that you'd need to find in order to complete your monster logbook.
 * Circle of the Moon featured the Trick Candle, an enemy that also only appears in a dead-end room previously occupied by a boss. Although it looks almost identical to a regular candle, whipping it causes an enemy to fall to the floor and disappear. You have about two seconds to kill it for its very useful magic card, which allows you to summon baddies.
 * Dawn of Sorrow has three enemies which can only be found by deciphering tabloids you find lying around. One will only appear if you use a certain weapon in it's area, at which point it will jump into the foreground and almost immediately jump back, meaning you have about one hit to kill it.
 * Order of Ecclesia has the Sasquatch (which is basically the same monster mentioned above). Once it sees you, it panics and leaps into the background. Getting a picture of it for a villager mission is easy enough, but killing it for the Bestiary is another story.
 * In the Capcom-produced Zelda games (The Legend of Zelda Oracle Games, and The Legend of Zelda the Minish Cap) there are extremely rare "Golden" versions of regular enemies, which not only have far more HP than regular enemies, but also move at crazy speeds. They only appear after certain events—but when defeated, they drop buckets of Rupees. They also appear in Freshly-Picked: Tingle's Rosy Rupeeland.
 * In The Legend of Zelda Ocarina of Time, one of the sidequests involves you searching the whole of Hyrule Field for Big Poes. You can only find each of them in very specific spots, you have to be riding Epona, and you have all of 5 seconds to kill them with arrows before they vanish and you have to try again. Hope your aim is good.

Driving Game

 * Need for Speed: Most Wanted has Sergeant Cross himself. A normal undercover Corvette will net you around the $20,000 in bounty. Destroying Cross's car, however, will net you a whopping $200,000!
 * Depending on the circumstances under which you take out Cross's car, it is also a Crowning Moment of Awesome.

Fighting Game

 * Cross Edge has Lujit, a small, pink, Level 120 monstrousity from hell. It appears in a few normal dungeons, is about 70+ levels above what you will probably be at first potential meeting area, has ridiculous HP, sickingly large range attacks, does ludicrous damage, is very fast, almost impossible to run away from, regenerates, can take a metric ton of moves a turn, and if it gets first attack can clear your group in a single round. However it does have horrible defense and is VERY valuable to kill.

First-Person Shooter

 * Left 4 Dead 2 has the Fallen Survivor in The Passing. They have as much health as a Witch, making it hard to take down. They are also fire proof and they always choose to run away (although sometimes there's no good escape options). The reason you would bother fighting this zombie is because it can hold a mix of either first aid kits, pills, pipe bombs, or molotovs. A single hit from a melee weapon is still enough to take them down, but their habit of running away usually puts of wall of zombies between you and their wonderful loot. Especially annoying because sometimes they're just carrying two grenade items... which would normally be nice, but The Passing has lockers filled with infinite supplies of grenades scattered throughout the first two chapters (the final chapter has neither the lockers no the Fallen Survivors).

Hack and Slash

 * In Drakengard, there is one enemy per level that rises out of the ground when you enter his proximity, glowing either red or blue and switching between those two colors. You have roughly a minute or two to defeat this enemy. Only physical attacks can harm him when he glows red, and only magical attacks can harm him when he glows blue. If you defeat him, he drops an item which adds a certain amount of experience to all the weapons you have in your Hyperspace Arsenal.
 * Gauntlet (1985 video game) has the Thief, a very fast enemy who runs up to your strongest player and steals potions, keys or even points from him/her. If you manage to kill him you can loot up a 500-point gold bag from his body; otherwise you'll have to get back your loot on the next dunegon level.
 * Fugitives in the third Sengoku Basara game. Each map has one, they have a random chance of appearing every time you start the map, they appear on a few predetermined and out-of-place locations on the map (and unless you bring a max level Hattori Hanzo, you won't know where) and run away about a minute after appearing. Fugitives only block and counter and have an extremely powerful block, making them impossible to kill without using a Basara art, Hero time or by loading your weapon full of Golden Centipedes. Each map's fugitive grants a permanent attack, defence or luck bonus when defeated, but only once for each character.

Maze Game

 * In Wizard of Wor, the Worluk appears once per level, moves quite fast, and can escape through the side doors. If it is shot before escaping, points will be doubled on the next level.

MMORPGs

 * Don't even start on the monsters that drop the Ultra Rare Items in Kingdom of Loathing. The odds of encountering one are so slim as to be meaningless. For each of them, one player per day gets a shot at the fight, and it's determined by some byzantine step-measuring system that gives that one shot to the first player who's there during the right time of day and has the step counter that no one's allowed to see in the exact right position. In theory. Wait, no, that's how it used to work, and now nobody's got a clue. Essentially, if you're aware these things exist but haven't been informed that you'll never see one, you could waste weeks on one of these before giving up. Most veteran players have never seen one; fighting one is a once-in-a-lifetime event unless you search obsessively (read "search for it with all your turns every day for years").
 * At least one person had defeated an Ultra Rare and come away with nothing, not even a consolation ribbon. The Black Cat familiar, which is specifically designed to hinder you has many wonderful effects one of which is randomly destroying dropped items at the end of combat. Ultra rare items are not immune to this, at least one person has had the cat knock a one in a billion item down the drain.
 * Ultra Rare monsters are also hard-coded to win initiative, and a few of them appear in early zones where they are considerably more powerful than normal monsters the player may be equipped to fight.
 * One of them looks and fights the same as a common monster, except it has an animated avatar; better hope you don't run away from it by mistake...
 * There are several notable Metal Slimes in zOMG. The first is the Ring Box, which is a rare version of the Pink Giftbox Monster (which in itself is a Chest Monster). They fly very fast, and have higher HP than normal Giftboxes. Defeating them nets you several rare rings. The other Metal Slime is more of a Bonus Boss. The Landshark appears randomly, deals a ton of damage, and is one of the only CL 10 monsters in the game. But it also drops the Surfboard Fin loot item, which is extremely rare.
 * 11.6 CL. The landshark has 11.6 CL. That's higher than the player.

Platform Game

 * Gold Beetles in Sonic Adventure 2 only appear for a brief moment in one specific part of each level, and vanish just as quickly. Destroying it nets you a huge amount of points for your score, which go a long way to earning a better rank for certain level requirements.
 * Golden Mice and certain rare blue ghosts in Luigi's Mansion. They all appear only in certain areas, after doing some random and senseless task, such as coming into the corridor from a certain room, examining a certain lamp a number of times, getting hit by the bat that is next to that lamp and then going to the place where the mouse spawns, and if you don't catch them right when they appear (they move like greased lightning) they'll be Lost Forever...but if you manage to suck them up, they'll drop tons of money.
 * The Starbag in Super Mario Galaxy is encountered in only a few areas, and at first you can only see its footprints traveling across the ground. Approaching it causes it to run away from you. If you spin-attack just ahead of the footprints, you'll expose the enemy, who will get stunned. You can then defeat it with another spin-attack to get a shower of several dozen Star Bits.
 * UFOs in Kirby's Adventure take on shot at you then fly off screen - and unlike every other enemy in the game, if they escape or are killed they won't return until the game is reset. Eating them, however, gets you the powerful UFO ability.
 * Also, Golden Waddle Dees from Squeak Squad and Sneak Sacks in Epic Yarn. The former is unreasonably fast and will sprint straight over the edge of a bottomless pit if you don't trap it quickly enough, while the latter moves at a speed slightly faster than Kirby can run at and will disappear if not killed in time. They leave behind treasure chests and dozens of beads, respectively.

Real Time Strategy

 * Iridescent Flint Beetles in Pikmin are invincible, randomly appear, scurry around, then burrow back underground. But if you can make a thrown pikmin land on its back, it will drop pellets and nectar. Skills players can get multiple payouts from the same bug.
 * The sequel keeps the Flint Beetle and adds two cousins. The Iridescent Glint Beetle has a shiny golden shell and drops a Treasure or multiple sprays when hit. There is also the Doodlebug, which can drop sprays, pellets, or nectar, but periodically releases poisonous gas that can kill Pikmin.

Roguelike

 * ADOM has the fast and powerful giant boars, which are only encountered in the wilderness, where it is possible for enemies to flee. If killed, they sometimes drop a giant boar skull, which is required for an Ultra ending.
 * Filk, the quickling bard also in ADOM fits the bill. He lives in (usually) a deep level of the infinite dungeon, has the highest speed in the game is extremely good at dodging your attacks. Killing him is also required for an Ultra ending.
 * Elona's Golden bell, plays this trope completely straight.
 * Smeagol in Angband. He moves fast, is hard to hit, and can teleport away after stealing gold (or, in previous versions, items). Very difficult to kill when you're first likely to encounter him. Wormtongue is not as fast, but is also very difficult to hit when you first meet him, steals your gold and items then teleports away, and makes traps. This means that you might kill him, then fall through to the next level before you pick up the awesome artifact he just dropped. Hope you have preserve mode set. Many of the thieving monster in this game fit this to some extent.
 * Jetpack Santa Claus in Toejam and Earl will occasionally appear, fussing over his sack. If you're lucky to see him from far away enough, you have a chance of sneaking up on him from behind (a mechanic no other enemy in the game uses) and get a few presents for your trouble. Otherwise, off into the wild blue yonder he goes.
 * In Z.H.P. Unlosing Ranger vs. Darkdeath Evilman, there are Metal Cacti, enemies that run away really fast, and pretty much REQUIRE you to corner them to get any hits in.

Role-Playing Game

 * Named for the Metal Slime (or Metaly as it's called in some games) from the Dragon Quest series. It's actually a regular old blue Slime with metal coating, but defeating one of them will get the party (or just the player in the first one) lots and lots of experience. Actually doing so is a pain because despite its low HP, its defense is high enough that some attacks will outright fail, it is completely immune to magic, it's also lightning fast, and as noted, it likes to run given half a chance. It is not uncommon to encounter a large group of MetalSlimes (and/or it's relatives) and have all of them flee before your party can kill even one; in fact, it isn't even that uncommon to encounter a number of them and have every last one of the cowards bolt before the party can even act. Other variations of this monster appear in the later games, such as the Liquid Metal Slime (known as the Metal Babble or Metabble in some games) and the Metal King Slime.
 * The Platinum King Jewel deserves a special mention here- not only does it have the highest HP, defense, agility, and XP of the Metal Slime family, it also can use Dazzle to blind your party, making them miss so much that doing any damage to it is virtually impossible.
 * Amusingly, the Metal King Helmet is so strong that it's even better than the Legendary Hero's helmet, which is part of the armor you spent the better part of the game looking for. Many of the Dragon Quest games have Metal Babble/Metal King equipment as infinity-plus-one equipment, aptly named after the creatures with enough durability to take only 1 damage from most attacks.
 * The Gold Golems (originally Goldmen) are slow, but powerful by nature, and provide the most money out of any creature in the game. As of Dragon Quest VII, the amount of gold they drop has been eclipsed by the Gem Slime (formerly Gold Slime), a relative of the Metal Slime who acts much like any other Metal Slime. However, the Gem Slime can also use Magic Burst, which is the deadliest spell in the game, so if it doesn't flee immediately, it can deal serious damage to your party.
 * The Dragon Quest games also feature, at least in one game, a Metal Slime with an incredibly rare random drop: In Dragon Quest III, the Metal Babble (or Liquid Metal Slime), which has a 90% chance of running every round, an agility high enough that it almost always gets first attack, and a defense so staggeringly high that often nothing but a "tremendous hit" will take it out, has between a 1 and 5% chance of dropping the Happy Shoes, extremely valuable footwear that gives the wearer XP for every step in the field, which makes leveling in towns entirely possible. Needless to say, hours upon hours upon hours can be invested trying to get those shoes.
 * In Dragon Quest III there's a magma cave area where Metal Slimes are so common it is possible to find yourself swarmed by up to seven of them at the same time. You probably won't be able to kill all of them, mind, but with so many of them, its very easy to off at least one or two before they all run away.
 * Fun fact: In III, the spell "BeDragon" (which will turn the caster into a dragon,) still does normal damage. So while it takes two turns to do any damage (and several slimes will have run by then,) the rest get totally burninated.
 * If your other caster in the party casts the agility spell on the bedragoned character it will be fast enough to act first before any of the metal slimes, torching up any of those who did not flee in the first turn with 100% certainty.
 * Likewise, Dragon Quest IV had the Royal Crypt, where you could get swarmed by Metal Babbles (which gave 7.5x the XP of Metal Slimes). However, in this game you had the lovely BeDragon spell, which could hit all enemies with a never-miss, defense-ignoring flame attack. So, while you had to spend a round casting it, the next turn any Metals that were still around were dead. Nothing like getting 50k+ experience from a random encounter. Unfortunately, the DS remake nerfs the equivalent spell Puff (not to be confused with Puff-Puff) so that the metal blobs are immune.
 * Most Metalys encountered in the first Dragon Quest Monsters have an attack which deals heavy defense-piercing damage to a random Mon on the field. Including the Metaly using said attack. Suddenly they're not so hard to kill.
 * Metalies were in fact not the penultimate metal slime in the game. Two breed together got you a Metable (a melted version of the metal slime, a palette swapped Bubble Slime, "Babble"), and breeding two of those got you a Metal King, breeding two of those got you a Gold Slime. Now, doing this breeding the normal way meant your Gold Slime has all of 1 hit point (parents stats averaged, starting with a maximum of 8 HP), but there was another way to get a Metal King (breeding Metal Drake—a mechanical dragon—and King Slime) which could result in a 400 hit point monstrosity that could never take more than 1 damage. And then you feed it the +max HP food items. Being a 14th (or higher) generation Mon it tended to learn every special move in the game, of which you got your pick of 8.
 * In Dragon Quest VI, there is a hidden job class that allows the player to take on the characteristics of a Metal Slime, i.e low HP, high spell resistance, etc.
 * Similarly in Dragon Quest VII, PlatKing is the ultimate monster job class.
 * Notably, several games offer special weapons or abilities specifically designed to guarantee 1 or 2 points of damage against metal slimes.
 * Dragon Quest IX introduces the Metal Medley; Three MetalSlimes stacked on top of each other. This particular flavor serves as the step between the Metal Slime and the Liquid Metal Slime.
 * In Dragon Quest Heroes: Rocket Slime, where you play as a slime, the role instead goes to the Goodybag: kill 'em to get lots of gold, or send enough of them back to town to recruit one as a very effective crew member for your tank. It also lampshades the traditional metal slime; get an imp in your town and it'll chase the liquid metal slime character to try and kill it for experience (the liquid metal slime doesn't mind too much, and even likes being able to practice its running).
 * Shining in the Darkness features the closely related "ooze" family of monsters, the last of which is the oh-so-shiny crystal ooze. Satisfies virtually all of the listed criteria for this trope at the same time; appears only on the final level (and even then, is rather uncommon), inflicts heavy damage and attacks quickly (and can call other monsters for assistance), is prone to flee from battle about 25% of the time, and has only a few HP, but also extremely high defense; high-level magic typically inflicts no more than one point of damage (if any), and physical attacks usually do no damage at all. The game offsets this by providing a number of "instant kill" items which the oozes cannot avoid, but these items themselves are "secret" and difficult to find. The reward for killing a single ooze is a whopping thirty thousand EXP, and they can appear in groups of half-a-dozen or more, resulting in the potential for multiple level-ups for even experienced adventurers; the sheer numbers involved reportedly cause the game to become unstable for players who even encounter the monsters.
 * Mega Man Battle Network has the Rush virus, which has low HP, only takes one damage per hit, and will flee if not defeated in time. It pops out of the ground, and needs to quickly be struck before it submerges again, much like a game of whack-a-mole. If you do beat it, you get a useful chip that makes you invincible as long as you aren't attacking, which lasts much longer then the standard form of invincibility from the Invis chip.
 * The Cactroct/Cactuar from Final Fantasy VI onward. They're generally rare, prone to running away, extremely difficult to hit through conventional means, and will reward tons of EXP, AP, gil, and/or valuable items if defeated.
 * In Final Fantasy V, there is a silver squirrel, named "Skull Eater", in the Jachol Caves. Despite having only 1 HP, it has a very high defense, and its attacks can be very damaging if your party is at a low level. Also, if you cast magic on the Skull Eater, five more Skull Eaters will appear. If you defeat it (or it runs away), your party will receive 5 AP. The strategy for Skull Eaters is to use the Beastmaster's Control skill, where you can command it to eat its own skull.
 * In Final Fantasy VIII, the Cactaurs inhabit a tiny island you can't access without a plane and wouldn't think of landing on without a guide. They are also incredibly fast and hard to hit and would just as soon run as look at you: fortunately enough, all of Squall's gunblades have maxed out accuracy and thus never miss, as does Selphie's best weapon, and with the Initative passive skill, you can always be more or less guaranteed to kill at least one. For doing so, you get a whopping 20 AP in a game where more than a few bosses give just 5 or 10 AP. This is essential if you're trying to keep your levels low because they only give a tiiiiny amount of EXP.
 * Flowering Cactuars in Final Fantasy XIII fit this trope perfectly: they're only encountered in a relatively out-of-the-way location, (one only accessible during a certain sidequest, and even then, you have to take a specific path) flee from battle at the first opportunity and give 15000 CP when killed. (And bearing in mind they always come in groups of 3 or more, that's a pretty big deal.) However, if they don't flee, they'll bombard you with devastating "-Ga" magic, quite capable of a Total Party Kill. What makes them unique among other examples of this trope is that they're actually very easy to kill (they have one of the lowest stagger thresholds in the game), it's just that it needs to be done VERY quickly: in literally 5 seconds either they'll have fled, or you'll be dead.
 * Cactuars aren't the only ones: 5, 6, 7, and 9 at least feature movers—tiny, fast, absurdly evasive, and prone to escape at the drop of a hat, but if you can kill them they're worth the AP; then of course there are magic urns, with a trick. Both of which predate the far more prolific Cactuar.
 * Final Fantasy IV: The After Years also introduces a new type of Metal Slime into the series: encountered only in a specific floor of Palom's Bonus Dungeon, Gil Birds are a golden version of the usual bird enemy with high speed, third-highest HP out of any enemy or boss in the chapter, the usual tendency to run away often, and a large variety of attacks, and as can be expected from the name, they drop either 999 or 9999 Gil when defeated depending on the moon phase. They're not particularly deadly though, but hunting for them is worthwhile due to the large amount of items that will end up Lost Forever if you don't buy them during his chapter. To say nothing of the various "Clubs" that Palom can go to if he has the obscene amount of gil necessary.
 * Some games also have treasure chests appear in battle. Not strictly metal slimes, but they certainly are rarer than most enemies.
 * The Pokémon series has no true Metal Slimes, but it does have a number of Mons that have the ability to escape from battle before you can capture them:
 * The Abra species are commonly encountered and have fairly high catch rates (comparable to Com Mons, in fact), but their sole combat skill is to Teleport out, ending the battle on turn one. Unless you can put it to Sleep before this or use a skill or ability to block its escape attempts (Mean Look, Arena Trap, etc.), you can only attempt to catch it at full health before it runs off. Abra also comes with a lesser extent of Magikarp Power, learning no actual attacks without the aid of a TM (Teleport being their only naturally learned move) until it evolves into Kadabra.
 * All Pokemon encountered in the Safari Zone can flee from battle at any time, and your only options besides attempting to snag them at full health are to throw bait (making them stick around, but more difficult to catch) or rocks/mud (making them easier to catch, but more likely to flee). This goes double for ones like Chansey, which are rare encounters even in the Safari Zone, and have a low base catch rate—just like the Pokedex entry says, catching one successfully is a Luck-Based Mission. At least some of them can be caught in non-Safari areas, where you can wear them down and capture them normally. The first generation's Missingno glitch could also be exploited to encounter Safari Pokemon in normal battles.
 * Starting in Generation II, some legendary Pokemon roam the region, and when encountered, they immediately flee from battle. However, their HP doesn't regenerate between battles, so catching one involves wearing them down through repeated encounters.
 * When hunting for Entei, Raikou, or Suicune, even if you do use Mean Look to prevent them from escaping, they can just blow you out of battle with their Roar.
 * Even worse than it sounds: In some generations, if they use Roar, it counts as you having left the battle, not the Pokemon that used it. If you leave a battle against a roaming Pokemon, the Pokemon is Lost Forever, meaning they can essentially disappear from the game at will,at least until Gen IV, that is. In more recent games, mons that are knocked out or ran from will respawn after defeating the Elite Four the second time. However, if you fail again, you're out of luck unless you have a Game Shark.
 * Shiny Pokemon of any species that learns moves like Roar, Whirlwind, or Explosion. Because, yes, they will invariably use the battle-ending move. Especially frustrating are shiny Pokemon encountered in the Safari Zone, as they combine two forms of rarity at the same time.
 * Crowned Monsters in Romancing SaGa (only appears after long rounds of trading with a specific class of monster).
 * Gold-plated enemies such as the "Wealth Hand" and "Treasure Hand" pop up at certain intervals in Persona 3. They always get first strike, run away at the drop of a hat, are immune to all magic and have impossibly high dodge abilities and defense. The only way to kill them is to use a physical attack they're weak against—they switch weaknesses every round and analyze never works on them.
 * And you'll need to hunt and kill these little cretins, for completing Elizabeth's requests. In the Updated Rerelease Persona 3: FES, the Hands now occasionally drop "empty" weapons, which are used in the new item forging system.
 * By far the worst part is that, when you find them before entering the battle, you have to sneak up on them. Failure to do so (and sneaking is extremely picky) will result in the creature running away from you just as fast as you run after it. And if you manage to corner it? It disappears.
 * In FES, they also did away with their immunities, which makes hunting them marginally less irritating. They're horribly annoying to hunt, though, because of their tendency to run away at the slightest provocation.
 * They show up again in Persona 4, where they're usually immune to physical damage. There is a trick to them this time around, however—they're always weak to the element used by the person who owns the dungeon, and the ones in the last three levels are vulnerable to dark and light. The sole exception is the one in the Heaven dungeon, who is immune to everything. How do you beat it? Use any Almighty-type attack (including an All-Out Attack), or break its resistance—both options are easier said than done.
 * In the first Persona, there's an incredibly small chance you'll run into series staple Alice in dungeons. She lacks real offensive capabilities, but has the best stats this side of the final bosses... and she'll usually run away instead of using any magic. If you manage to defeat her, she'll drop a Pagan Idol, which is used to fuse Beelzebub.
 * Loopers from Skies of Arcadia. They actually mildly subvert this in that you'll encounter them anywhere in numbers of about one or two per battle, but the location where you encounter nothing but them (usually in groups of five to seven) is near-inaccessible. Also, they have a good chance of dropping Moonberries and will occasionally drop an item that, when equipped, permanently stops enemies from being able to run away from you ever again. The obvious problem is that you have to beat some of these guys first before you can get a hold of that. Magic of the appropriate element, Rain of Swords, Cutlass Fury and the Skull Shield's counterattack are all options.
 * The Gold Eggs from Breath of Fire 3--
 * And the Goo King, of course. They have a very small chance of being encountered in one specific area of the final dungeon, always run away (unless you, and if you do they start casting Ragnarok every round), and drop the Goo King Sword.
 * The Kudan in Shin Megami Tensei spin-off game Devil Summoner: Raidou Kuzunoha vs. the Soulless Army is one such enemy. It not only runs away, but it randomly switches what types of attacks it is weak and immune against, making the act of defeating it very frustrating. Worse, in Persona 2, a Kudan will often show up, depower any Personas your party has equipped to Level 1, and then disappear.
 * Kudan in Devil Summoner also ABSORBS gun attacks (i.e. Raidou's revolver) and can fully heal itself with the best single-target healing spell available in the game. This editor has yet to actually kill one of these bastards without Belial (who is the first familiar that can learn the best combination attack in the game, "Astral Burst")...and Raidou has to be Level 66 to be able to fuse and summon him.
 * All Shin Megami Tensei games had a group of monsters known now as Fiends. Exceedingly rare, exceedingly powerful, they could show up at any time in certain dungeons and wipe out a party. Subverted Guide Dang It because, despite the hidden nature, they were purposely left out of guides for the longest time, to fuel the mystery. (Ironically, they throw themselves at you in Nocturne.)
 * This trope is parodied in Mother 3, the sequel to EarthBound. At one point, you have the chance to fight the rare Metal Monkey. It's got stupid high defense in order to compensate for its low HP, but instead of giving the player a lot of experience upon its defeat, the game instead tells the player that, roughly, 'they had a good experience.'
 * There are regular Metal Slimes in Mother 3 as well. Chapter 1 has the Soot Dumpling, an enemy with very high defense that can only be beaten with two 16-hit combos. If you don't kill it quickly enough, it blows away in the wind. Later on, you'll find the Black Beanling, which has powerful PK Fire attacks and can run away to boot. Beating it gives you a whopping 16,000 experience points. There's also the Top Dogfish, which gives 800 DP and always drops Meteotite which can be sold for 2500 DP, effectively making it the richest enemy in the game. It's very rare like the others, but it's very tough and hits hard, so it blurs the line between this and Boss in Mook Clothing.
 * Also, in Earthbound you can wander through any of the two deserts present in the game and beat the Criminal Caterpillar or the Master Criminal Worm (two randomly encountered enemies) in order to get A LOT of experience points. Even though they can easily defeat your party if they get to fight, most of the time you'll end up catching them from behind, getting an automatic victory.
 * Mother 1 has the Red Snake, which usually runs away the first chance it gets.
 * And of course there were Starmen Super in Earthbound - appearing only in one location, having the ability to teleport, being stronger than usual enemies there and indistinguishable from normal Starmen without entering the battle. To make matters worse, they were the only chance for Poo to get a weapon and they dropped it extremely rarely... and they were impossible to encounter after defeating the area boss.
 * The Digital Devil Saga games on PlayStation 2 both have highly annoying Metal Slimes in the form of Omoikane. In the first game, it has sickeningly high HP and immediately flees on its first turn after you attack it. The second has them showing up in groups of five, each one weak to a single magic element which the others are immune to, and will self-destruct for huge rewards if you hit the weakness; unfortunately, since they still have positively nauseating HP, and they just call you a cheater and flee if you take the obvious route of using multi-target spells, your only option is trial and error - which you only get two shots at unless one is the right guess, thanks to the game's unusual turn structure. Both of these are made much less egregious by having a chance of showing up everywhere, even.
 * Three enemies in Chrono Trigger qualify... Though they go by different names and use different sprites, they are functionally identical. They have enough HP to (usually) take two hits before dying, use 'Lock All' (also called "Omnilock") to disable every combat function except basic attacks (which takes effect regardless of any status-locking items you may be wearing), and have such an absurdly high dodge (despite being rocks and turrets) that hitting them is basically luck. To add insult to injury, they run after a short time. They also don't EVER reappear, so you only get one chance per encounter. There is one Rubble near the middle of the mountain on the right side that will respawn. But only the one. Defeating them grants 1000 Exp and 100 Tech Points; the XP is above average but not insane like the TP is. Another area introduces a palette-swapped version of the Son of the Sun and his little winged eyeball buddies. The smaller ones run away after enough time passes, and if you ignore the big one and go after them, he starts punishing you with a small version of Flare. Not terribly damaging at higher levels, but damned annoying. Especially in the point of the Black Omen where you face two of the bigs and four of the littles all at once.
 * The expanded DS version adds another. Instead of using Lock All/Omnilock, it fights back instead, but it still runs on a whim and pisses you off royally. It also appears totally randomly, instead of being a set spawn that never comes back. They're also the only things that can drop a certain material for having an item crafted. Of course, you can just have Ayla Charm (steal) it, too, which ups the chances of getting that item greatly.
 * In Final Fantasy IV, the Pink Puff enemy (Flan Princess in the GBA remake) appears very infrequently in the final dungeon, and is very hard to beat before it runs away. If you succeed, you MIGHT get a Pink Tail, which you can trade for a set of armor so strong it'll make the equipped character practically invincible.
 * Meeting the Pink Puff is a 1 in 64 chance in a dungeon with reasonably strong monsters. It will also only show up in a specific room, making this, from the get-go, a Guide Dang It. Once you actually meet and manage to kill a Pink Puff, the drop rate of the Pink Tail is, again, 1 in 64, multiplied by the 1 in 20 chance that they will drop anything at all. Admittedly, they attack in groups of five, and each one has a 1 in 1280 chance of dropping the Pink Tail, but the laws of probability work in such a way that the odds of getting a Pink Tail from a fight against the Pink Puffs are around 1 in 256.4, so for every battle in the specific room in which you can encounter Pink Puffs, the chance of getting a Pink Tail is roughly 1 in 16,410. Needless to say, even if you know about it, it's usually much easier to just level grind to the point where your characters are practically invincible anyway.
 * This is made slightly easier in versions where the Siren item (or the Alarm item, depending on the translation) was not dummied out, as it can summon a group of Pink Puffs in the single room where they can appear. They still have a very low chance of dropping the Pink Tail, though. In some of the earlier versions of the game, by the time you reach the Pink Puff room, the Sirens can no longer be bought or stolen from enemies. You have to stock up beforehand.
 * In the original SNES version and some of the early remakes, since each enemy has a 1 in 20 chance of dropping an item and therefore a fight against multiple enemies can result in multiple item drops, it is technically possible to get two or more Pink Tails in the same battle (the odds of this are nearly 1 in 82,000). But not in the DS remake, where the item drop slots for Pink Puffs is exactly 1 - so you will never get more than 1 pink tail from any set of Pink Puffs. Worse, they can possibly drop several items (including the pink tail) and if a lower probability drop is given by another pink puff, it'll override the rare pink tail. So, the only way to get a pink tail is to defeat all 5 pink puffs and hope only pink tails are dropped (one or more, it makes no difference).
 * The Legend of Dragoon has a whole series of these monsters; they have 4-6 hp but only take 1 damage per hit and will run if they can be killed before their next turn, meaning they can't be taken out until you have certain specific items from later on in the game. Not only that, they is insanely fast, getting multiple turns in before your characters do. The worst part is, they are practically necessary to grind so one can get the best gear in the game. At least when they do attack, they take off 1/10 of your maximum hp, so they really don't do THAT much—except for one variant of these slimes named OOPARTS, who can insta-kill one of your party members before running away. If it feels like it, it can even stick around and nail another character.
 * Cores in Lufia: The Ruins of Lore have high def/low hp, but most are slow, and there are plenty of attacks that can easily defeat them. The exception is the Anti-Core, which is insanely fast, capable of taking many turns before your party gets one, with "flee" getting a high priority. By using another monster with a confuse ability, it is possible to capture one, gaining a valuable ally (especially if you boost its dismal attack with power sources).
 * Cores in Lufia 2 are also very fast and usually they flee before the players get a turn. sometimes when rng is merciful one of them will not flee the encounter and player can kill it for huge amount of xp. Danger level of the encounters is pretty low so its it a very safe level grinding trick.
 * Many of the later Wild ARMs games offer the creature Grow Apple. Grow Apples have 1 Hp, don't have any attack, and run from battle as soon as possible. However, if you kill them, you gain experience points equal to the overkill damage. If you hit it with a 1,000 point blast, you gain 999 experience (-1 for its 1Hp). The creature Mega Apple (not to be confused with the game's curative item Mega Apple) is similar, but much harder to hit and offers 10x the overkill damage.
 * For the gamer looking for money, the games also provide Melchom. Elemental magic heals the creature, and in doing so, increases the money it drops by that amount x3. And unlike the aforementioned Apples, it isn't nearly as fond of running away. Or, if you want to make money much easier, just steal from Rat Monkey. The loot you pilfer off of him can be sold for 999,999 gella. However, a character who can steal is only available late-game, so the Melchom method is usually used before then.
 * There's also the Creeping Chaos enemies; while they're generally very difficult to defeat and don't run away, they appear very randomly and generally drop a ton of experience and/or Duplicators.
 * Kingdom Hearts:
 * The White Mushrooms, Black Fungus, and Rare Truffles from the first Kingdom Hearts game. All three appear only at a few set locations... maybe. All three have a chance of dropping extremely rare items if you defeat them in exactly the right way. That's it for the White Mushrooms and Rare Truffles, which are completely harmless. The Black Fungus, on the other hand, complete their Metal Slime status by gushing poison at you and periodically turning completely invulnerable.
 * Kingdom Hearts II features Bulky Vendors: a variant of Metal Slime. While they appear randomly, run away, and can't be killed by conventional means, they are killed extremely easy with Reaction Commands. Rather than giving large amounts of experience, they drop rare items. The rarity of the item is directly related to its health, which is constantly depleting as soon as it spawns. But there's a catch: it's relatively slow when it first spawns, but as its health goes down, its speed goes up, and this thing almost literally bounces all over the place. So you're better off looking for Orichalcum then trying to get one from a Bulky Vendor. Regardless, when you smack them with a Reaction Command they drop a crap ton of materials used for synthesis with their rarity depending on how much health they had when you used a Reaction Command. Also, good luck trying to hit one with the Reaction Command when it's got a sliver of health left; when they've got next to no health left, they are probably the fastest enemy in the game.
 * Bulky Vendors return in 358/2 Days, only without the reaction commands. This time, they have to be defeated quickly before they escape—which is much harder.
 * Re: coded has Gold Tricholomas, a gold variant of the other mushroom Heartless that only appears in System Sectors. They love to teleport away from you when you're trying to attack, can spew a poison cloud that can blind you on top of doing damage, and they'll flee the battle if you can't kill them quickly enough, but they drop tons of SP, which you can redeem for goodies like stat upgrades and equipment.
 * Super Mario RPG had Shy Guys in camo called Shy Rangers appear in one area of the game. Unless you have enough speed from armor and accessories, they'll escape before you can even act, and will still escape the moment their turn comes up (Unless they're poisoned, which makes them fight to the death for some reason). They don't even drop anything or give enough EXP to make it worth going through the trouble—unless you throw a Yoshi Cookie to make Yoshi consume them; then you get a KeroKeroCola, the most powerful healing item in the game.
 * Paper Mario and its sequel Paper Mario: The Thousand-Year Door have Amazee Dayzees, Palette Swapped versions of the more normal Crazee Dayzees. They have ridiculously high HP, the most damaging single attack of any Mook, high defense, and to top it all off, they put you to sleep. Luckily, in the second game, a skilled player can defeat them in one hit by effectively utilizing the Art Attack move—and they drop gobs of experience points; this is especially helpful in the later levels of the Bonus Dungeon, which is full of them. And you heal on leveling up! Also in the second game, it is quite possible to run into them at a point in the game where . Thanks, Nintendo!
 * In the first game, equipping the Dizzy Badge (a badge that causes Dizziness if you press the Dash button and collide with an enemy while dashing) and using it on Amazee Dayzee will make it dizzy, giving you a free turn—it can't run and it can't attack. This makes them easy to the point where the only real challenge is finding more to milk for all the experience they're worth.
 * Amazee Dayzees also show up in Super Paper Mario. Due to the mechanics of the game, they work differently, but they still fulfill the role; they have a powerful attack that puts you to sleep, but more often they'll turn tail and run at stupidly fast speeds as soon as they spot you, then flip between dimensions, forcing the player to flip as well if they want to pursue them. They give a lot of points, and drop Golden Leaves, which can be turned into Gold Bars, which can be sold for 100 coins, in a game where 999 coins is the maximum you can carry. They can also be turned into cards and sold for a 200 coin profit.
 * The World Ends With You's Pig Noise. The pigs have high defense, and run after about ten seconds. Later variants can be invisible, invulnerable to all but one specific pin, or need to be erased in a specific order. At least they're easy to find.
 * One of them even requires you to
 * Also, there's a Bonus Dungeon filled with them, Pork City.
 * In Golden Sun: The Lost Age, phoenix-type enemies fall into this trope. They're not too difficult to kill, but they're fast, have very powerful fire magic, and tend to run away. They also give many times as much experience points as other enemies in the area, and some will rarely drop Dark Matter. Oh and they move two to three times per turn depending on the species so three chances for it to run.
 * On the plus side, finding two of them at once lets you get experience point totals in the five- to six-digit range if you let them keep resurrecting each other.
 * In addition, there are the Djinn. About half of them join without a fight. The other half either require you completing a puzzle or finding them in random encounters on certain areas of the world map. Each of these creatures is the embodiment of one of the four elements, meaning they have some of the strongest abilities of that element (fire blasts for the fire ones, extreme healing for the waters, etc.) and, of course, can also run, requiring you to do the puzzle again or keep searching the map. However, defeating them is definitely worth it, as you keep them permanently, they give you stat boosts when activated, allow you to use a unique attack, and can combine with others to unleash powerful summons.
 * And of course, you need all of them anyway if you plan to access the Bonus Dungeon, fight the Bonus Boss and receive the Bragging Rights Reward.
 * River City Ransom EX brings the Entrees gang, who are all named after food items, and come rushing into battle with... weird weapons. They give out about five times more money than the game's most powerful gang, the Plague, but are also the quickest to run away.
 * But it's so worth it to get the chance to beat the crap out of someone by swinging a ladder.
 * Ghosts and spirits in Wizardry VII toe the line between this and more conventional Demonic Spiders, being very resistant to damage of all kinds, wielding the most devastating Standard Status Effects, vanishing at the drop of a hat, and late in the game, dropping Gorror Keys, necessary for challenging the Bonus Bosses... and thus getting the best loot in the game.
 * The Silver Kelolons from Lost Odyssey. Give out huge amounts of XP & some fairly rare pieces for the game's Item Crafting system, but nearly always run away & have tremendous defense. Fortunately there's a trick involving the defense-ignoring spell Gamble & the Casting Support skill that allows you to waste the little fuckers just about every single time. Coupled with the fact that there's an area where the only enemies that appear are Silver Kelolons & one other type of monster & you get a relatively painless way to level your party up to obscene heights in fairly short order.
 * And then there's the Diamond Kelolons in the DLC only Experimental Staff Remains. They don't run away 100% of the time, as they may choose to cast Death with a 3-turn charge-up first. However, that 3 turns window isn't much when they're immune to all magic and items, and complement their expected high evasion rate with a high chance to counter physical attacks, negating the damage. You can increase your chances with the Counter Seal skill, which allows a character to strike an enemy once without activating its counterattack but it remains a Luck-Based Mission trying to kill one. They're a guaranteed level up until about the mid 90's, and give the most SP in the game.
 * Gold Koopeleons in the second Mario & Luigi game fit this trope to a tee. There's also the Golden Beanies in the first game, which only appear in one early area of the game and actually appear to be an entirely different enemy at first, running away almost immediately after their guise is destroyed. They also drop a badge that, while seeming fairly useless at first, makes a later area of the game much easier.
 * Infuriatingly, the enemy that spawns Golden Beanies appears later in the underwater area. When you kill them there, however, the Beanies (and their golden variants) just float away.
 * The Lucky Minks from the last 3 Grandia games. Each one has an average of 15 HP, insanely high defense and evasion and a tendency to run from the drop of a hat. Grandia Xtreme had the coercion skill which prevented them from running, but killing them is still a challenge. Your reward? INSANE exp.
 * However, there's one time where, near the end of the game, they refuse to run, and instead fight back, and continuously call for reinforcements (they call up at least 10 of them), and it turns out they can hit for pretty decent damage, and with their high attack speed, this makes them adorable little nightmares. Suddenly, the EXP extravaganza turns into one of the most insanely dangerous battles in the game, as the very stats that make them difficult to kill turn them into an army of insanely powerful killing machines worthy of a bonus boss battle.
 * The Shiro Tail, a fluffy critter inhabiting the White Dragon Cave in Lunar: Silver Star Story Complete. Runs away a lot, drops a rare item and a ton of EXP.
 * Lunar: Eternal Blue had Ice Mongrels: two variants with each specific to a single location. Both had high dodge rates, a high tendency to run away and defense that rendered it near invulnerable to anything except the main character's sword skill attacks. The second type encountered actually gave 65534 XP divided amongst the party; if only the main character was in the party at the time (possible during the Playable Epilogue), any monsters defeated in addition to the Ice Mongrel would cause an overflow, and actually LOWER the total XP given for the battle (essentially negating the all of the XP from the Mongrel). Since levelling up never required more than 65534 XP, fighting them once powered up enough to defeat them reliably is a very efficient way to level grind.
 * Dungeon Siege II has the "???" (sic), basically a high-level thief who'll teleport in when your party has acquired some great items. He'll try to steal items from you, but if you attack him quickly enough he'll drop some good items himself, and rapidly disappear. It is possible to actually kill the thief (or rather, "defeat" him by getting his hit points down near 0) several times during the campaign, in which case he'll drop a series of items required to unlock the Developer's Room, which further gives access to some powerful ass-kicking gear.
 * Tales of Hearts have Apple, Peach, and Grape Gela. Found on one island in the game. Their HP is paltry, but they have 9999 defense and tech defense. They drop perfectly normal Apple, Peach, and Grape Gels, but their steal items are stat-boosting herbs.
 * Tales of Symphonia has the Bacura. "Hey, you can't fight the block thing in the mines!" Well, no, you can't fight it... in the mines. Instead, you have to fight it on the path by the SE Abbey, and due to their low spawn rate it helps to have a Dark Bottle active. It takes 1 damage from every single attack and runs away after a set amount of time. For defeating it, you get 11 to 35 Grade (depending on various things). For comparison, most enemies give you no Grade for merely defeating them (as the name implies, Grade is awarded for fighting well), most bosses give you 10, and beating the game gives you a whopping 1000.
 * They reappear in Tales of Xillia in normal, bronze, silver, and golden varieties. They can take the place of seemingly any enemy in certain areas starting early on in the game, and give a healthy sum of EXP. They only attempt to flee prior to engaging them in battle, but are surprisingly capable fighters for featureless blocks, and have sky high defense.
 * Demon's Souls has Crystal Geckos. These shiny blue lizards don't attack you at all, and will immediately run away once they're aware of your presence. If you're not quick enough, they'll run themselves straight off a ledge, or run into a wall and then burrow into it to escape. If you can kill one and loot its corpse, you'll get at least two different kinds of ores, the type depending on the level where you found the gecko. The best way to kill them is to use the Thief's Ring to make yourself harder to detect, be in soul form so your footsteps don't make noise, and snipe at them with a bow.
 * The Crystal Lizards return in Dark Souls, but they're much more manageable compared to their Demon's Souls counterparts.
 * Star Ocean: Till the End of Time has Mecha Scumbag, a more powerful version of the common Scumbag enemy that runs away when low on HP. Defeating one earns you a Battle Trophy.
 * They return in Star Ocean: The Last Hope as the Metal Scumbag, who can only be found once on a single floor of a single dungeon, and possibly appearing as an unexpected fifth wheel beside a group of normal enemies (there are about 30 of these on the floor and no way to guarantee which group he'll appear in)). Upon being seen, he immediately runs away. Killing him nets a boss-sized EXP increase, his Monster Jewel is capable of increasing Fol spoils by 25%, and Lymle has a Battle Trophy for killing 99 of them.
 * In A Witch's Tale, all card monsters love to run away and any attack (including ancient game breaking magic) will do 1 point of damage on them. Hope you score a critical hit. Or use one spell from March Hare doll.
 * The Forbiddens in Xenogears fit this trope nicely. They appear in a location that's accessible only temporarily, give you 2 turns to beat them before they break loose, and will drop Gold Nuggets or Gold Bullion if you beat them before they break loose. Oh, and they take a ton of hits to kill. About the only way they differ from a traditional Metal Slime is that once they break loose, they're actually pretty deadly.
 * Dragon Quest Heroes: Rocket Slime has a few metal slimes that you have to save, but you don't fight them. There is, however, a monster called the Goody Bag. It appears in every level, albeit in very specific locations, very rarely. (There is a mid-game level where it is the only monster that appears.) It runs from you when you see it, and Elasto Blasting into it will cause it to drop money. If you can catch it and throw it back to town before it casts Zoom and warps away, do this 30 times, and go see it in front of its statue in the Museum, then you will have a Disc One Nuke that warps right into the enemy tank, steals the ammo, and walks back to your tank. There are other characters that do this, but this one is the best because not only is it available early, it also has the most HP of these characters and is fast. (Also, if you go to the enemy tank at the very beginning, you can sneak in as the Goody Bag sneaks out with the enemy's ammo and break the machine keeping you out, saving the trouble of bashing the barrier over and over for a few minutes.)
 * In the game Boktai 2: The Sun is in Your Hand, there is a rare chance that one of the enemies will be replaced by a crimson version of itself. While not particularly nasty, and the same movement patterns as the generic version, the first time you meet one your damage to it will probably be 1 regardless of weapon, it will kill you in one hit and make you restart the area. It also drops "normally" a weapon that is several times stronger than anything you can find (or make) at the time, with its rare drop being a (potentially) Game Breaker item
 * Legend of Heroes VI and its sequel Zero no Kiseki have Shining Poms, which can dodge everything but S-Crafts and net you 1000 EXP per kill.
 * Final Fantasy VI Intangir... They spawn invisible, hasted, shelled, protected, and floating, absorb every single element, and are immune to every status effect in the game save for two: . If you manage to hit them, they become visible, and hit your entire party with a spell called Meteor, similar to the Lesser Lopros' Fireball. At the time you can first encounter them , this will 1-shot your entire party unless you have done an inordinate amount of grinding. Then they go back to being invisible again. They have 32,000 HP, and 16,000 MP. If their HP drops too low they run. And if you manage to kill them, they cast Meteor again, potentially on the entire party. And the reward, IF you manage to kill this monstrosity? 10 AP. The most AP you can get from any encounter in the game. Thanks to the Vanish Doom Bug, these could be farmed all day and night with Gau's Rhodox Rage without fear of them killing your party since this rages' special ability kills targets in a fashion similar to X-Zone/Banish, denying them a dieing action. However, with the FFVI Advance release, this bug has been fixed. Have fun.
 * Treasure Seekers fill this role in Diablo 3. If you hit them, they run away while gold spills out of the sacks they carry. If left alone for too long (while the player is fighting other monsters, for instance), it will summon a portal and vanish.
 * Hyperdimension Neptunia has the Metal Dogoo, the obvious parody. It does not drop any rare items and does not have as much HP (due to it being based on a variant of one of the game's Mooks), though it does drop a rather generous 10,000 Credits.
 * The Totokos (Who resemble a certain Studio Ghibli movie character) in Ni no Kuni give an absolutely huge amount of EXP or gold if you can defeat them, often much more then the bosses of a similar level, and also make good Imagen if you can capture them. However, they only show up occasionally, run away from you in the field, and will disappear if not caught quickly. Even after you catch them, you still have defeat them before they escape in battle, and the evolved forms of them frequently will use some manner of debilitating spell on the entire party to keep you from doing this.

Shoot'Em Up

 * Star Force has the mysterious picture of Cleopatra, hidden only in certain areas. Destroying it (which takes more than one shot) wins you one million points, but since the game is a Vertical Scrolling Shooter it could easily pass off screen first.

Survival Horror

 * Nemesis from Resident Evil 3: Nemesis meets the requirements. Nemesis is an uncommon encounter, rarely spawning, appearing in out-of-the-way locations a fixed number of times. Nemesis gives rewards; randomly-dropped unique (though not necessarily useful) items. Nemesis has a reasonably reliable way to end combat without the player being defeated or getting the reward through the normal "flee" mechanic, a special ability such as blinding him and knocking him off a ledge. Nemesis has Preexisting Encounters, so this trait manifests as Nemesis avoiding Jill in order to prevent the beginning of combat proper, such as outside the substation. And it is VERY difficult to obtain the reward before combat ends.
 * Resident Evil 4 has the Garradors. They drop a ton of cash upon their defeat. Upping the ante further, one Garrador that spawns in a cage actually guards a treasure chest with a very valuable piece of Vendor Trash in it, and predictably, it's pretty hard to get the item without killing the Garrador first. Garradors appear about four times in the entire game (one of which is a heavy Garrador). Unlike Regenerators, who also share the aforementioned traits, defeating Garradors (or even damaging them) is almost never absolutely necessary, to the point where running from them is easier than fighting them thanks to the Garradors' crippling blindness.
 * In the same game, Novistadors are also Metal Slimes. Earlier versions can turn completely invisible and have a one-hit-ko move that involves melting off your face, while later versions gain a pair of wings in lieu of being invisible, but still retains the same insta-kill move and are a pain in the ass to shoot. Their reward? Their eyes, which are precious gems that can be applied to another treasure you find which, if completed with all three different types of Novistador eyes, can be sold for an obscene amount of cash. The Gems themselves are also worth something too. They only appear in the bowels of the castle, and will completely disappear when you destroy their nest just a few scenes later, and the gem color is completely random (although there have been theories).
 * Also, the Dr. Salvadors. They are fairly uncommon, take alot of damage, can One-Hit Kill, and can be avoided most of the time, but taking one down nets you 10,000 pesetas.
 * Opoona has the Star Human. In addition to being run-happy, it has both extremely high defense and high evasion, meaning that even if you can hit it with a physical attack, it's not likely to do much damage. The best way to defeat it is to spam it with Armagebbon, and pray to all that's good that it doesn't flee. To add insult to injury, it has a one-of-a-kind drop attached to it, too (as well as a Rare Candy attached to it).

Wide Open Sandbox

 * The Eridians in Borderlands appear in the last few levels in the game, give you shitloads of exp, are the fastest enemies in the game and have insane shields, but pretty low actual health. What's that, Mordecai? One of your skills allows you to bypass shields completely?. Score.
 * Assassin's Creed II and its sequel Brotherhood feature pickpockets and Borgia Messengers who run away from you if they see you; if you manage to catch them, you get a hefty chunk of money as well as some rare trade items.
 * Terraria has "Pinky," a tiny, rarely appearing pink slime that drops gold coins—hundreds of times as much money as an average slime. It also has a lot of health, and if you hit it too hard, it may fly off the screen into oblivion. Have fun whittling.
 * Saints Row: The Third has Professor Genki, who can be found running around Steelport killing random people. He has tons of health but when defeated gives you tons of money and Respect.
 * Before the final release, Minecraft Slimes only appeared in first twelve layers of the world, four of which are full of unbreakable stone, spawn incredibly rarely, could only appear in one tenth of all chunks, determined on the world being generated, and frequently jump into lava or suffocate by spawning in spaces too small. The bigger ones also do enough damage to kill you very quickly if you have no armor, and split in two every time they are hit. On the other hand, the biggest ones can split into up to 64 Tiny Slimes, which each drop up to two slimeballs. Slimeballs happen to be incredibly useful for making piston machines (almost all types of machines are much simpler with Sticky Pistons, which can retract blocks in addition to pushing them) and are used in getting to the End and making several types of potions.

Tabletop Games

 * According to Gary Gygax, a similar creature appeared in the original Castle Greyhawk campaign he ran (not the version later offered by TSR). He threw a golden golem encrusted with gems (worth a small fortune if the thing were ever killed) at the PCs, but made it fast enough to always outrun them when fleeing.