Sprites and textures are expensive. Original ideas for monster types are even more so. As a result, there is a tendency to keep the number of distinct enemy types small. In an RPG or similar game where the player is expected to become more powerful over the course of the game, this is a problem, as the monsters stop being challenging about the time you Get on the Boat.
The solution many games go for is to have a small set of monster types, but have them appear with different graphics. Often, this change of design will be accompanied by a new adjective to go with their name (if the monster was based on a mythological or cryptozoological creature, subsequent names will be alternate names for the creature (Bigfoot to Sasquatch to Yeti), or the name of a similar creature (Cockatrice to Basilisk)). Typically, all such monsters will be vulnerable to the same strategy, or a variation thereupon, but later colors will tend to be more powerful. Elemental variations are a common version of this trope as are variations in size and adding or removing features like horns, wings, or crowns.
Underground Monkeys are often Palette Swaps, meaning only the colors change but models are recycled, but they don't have to be. As long as they're recycled versions of previous enemies, the changes between the different versions could be anything. You might have normal Goombas, winged Goombas, spiked Goombas... Even King Goomba is a type of Underground Monkey.
Results in the somewhat strange phenomenon that as you travel a diverse world, rather than simply seeing a diversity of creature types, you also see the same creature types, in a diversity of versions: in The Lost Woods, you find the Wolf, the Giant Rat, and the Forest Dragon; in the Slippy-Slidey Ice World, you find their icy counterparts Arctic Wolf, the Snow Rat, and the White Dragon; in the Temple of Doom, you'll face their dark counterparts, Dire Wolf, Plague Rat, and Zombie Dragon.
The most common Underground Monkeys are those whose names begin with one of fire, ice or lightning. In games which play Elemental Rock-Paper-Scissors, the colors may also indicate elemental weakness.
Contrast Artifact Mook, which is when an enemy appears in places and quantites that defy its original context.
Named for a Running Gag in RPG World, wherein the Underground Monkey is suspected of being attracted by Genre Savvy characters.
The Legend of Zelda: The Wind Waker and The Legend of Zelda: Twilight Princess avoid this with every enemy except for Darknuts, whose armour changes colour to indicate different levels of strength, and the various Chu types, the Zelda take on the classic slimes. There are some minor palette swaps as well, but these take the form of giving the enemies extra armour and better resistance to the player's special attacks.
In the The Legend of Zelda Oracle games, most enemies come in red and blue, indicating respectively a better attack and a better defense. This and various other instances of palette swaps throughout the series are nods to the very first game's use of two palettes for most enemies, usually a red/orange palette and a blue palette, where one would take more damage to kill than the other.
In The Legend of Zelda: Spirit Tracks, there are five types of rabbit you can collect, matching the environment in which they're found, including one that lives in the sea.
Chu-Chus in Twilight Princess come in a variety of colors, but not to differentiate strength. Rather, if you fill the remains of one in a bottle, it's color determines the effect of using it. If two different-colored Chus combine, the result is the default color, which was pretty worthless.
Ōkami has the same set of tactics with occasional additional attacks for the successive areas of enemies. Difficulty is achieved late in the optional extras with nigh-endless waves of enemies.
Games based on BIONICLE, since the toys are pretty much like this. "You are attacked by an Air Burnak." "You are attacked by a Stone Burnak." "You are attacked by an Ice Burnak."
Cave Story has a nice variety of enemy types, but recolors the critters (those beanbag-looking hopping things) and bats in several different caves.
While the main-series Kingdom Hearts games largely avoid this by simply scaling the strength of enemies found in later worlds, 358/2 Days plays it straight, with up to 3 different versions of many mooks where the only difference is size, a design choice that may have been mandated by limited space for the game data.
Resident Evil series has several: the Brain Sucker in 3 is an upgraded skin-swap of the Drain Deimos, the Sweeper in Code Veronica is a poisonous version of the Hunter, RE 2 has Advanced Lickers with longer claws and the ability to sense the player even when they are standing still, the Iron Maiden in RE 4 is a more demonic version of the Regenerator, etc.
Double Dragon did this. While it certainly wasn't unusual or unexpected for a game of the arcade era, the fact that all of your opponents were human meant that different coloured characters got rather stupid toward the end. Whilst a man with brown or pink skin made sense, the same character with better fighting skills and blue, grey or green skin later in the game was cause for raised eyebrows.
Dissidia: Final Fantasy: Every enemy in the game is a crystallized palette swap of the 22 characters you can use in the game, and all the bosses are those same characters, only normal. The only enemy that is at all different is the Final Boss, Chaos.
Metroid Prime 2: A lot of enemy concepts are recycled from Prime 1 with new models. Some were barely changed (like the recoloured Triclops) while others were given a complete overhaul, the Beetle becoming the much smaller Splinter, the Elite Pirate the Ingsmasher, Baby Sheegoths becoming Grenchlers, Chozo Ghosts becoming Pirate Commandos, etc. There's also a few examples in the games themselves, like the normal / ice / plate Parasites in Prime and the light / dark creatures in Echoes. The Ingsmashers simply reused the elite pirate combat codes from Prime 1 with only a small tweak being the shield thingy.
FPS games regularly do this with at least one of the more basic enemies (but tougher opponents sometimes get the same treatment). In the older era, this was done by changing the colouring of otherwise identical sprites, in 3D games it takes the somewhat more advanced and differentiating form of using different skins for the same model (or even different models for the same enemy). Common expressions of this includes;
Different weaponry and/or levels of toughness of the opponents (e.g. the processed humans of Quake II, the Cabal followers of Blood, Barons of Hell and Hellknights in Doom and Doom II).
Somewhat different abilities between the enemy types (e.g. the semi-invisibility of Spectres in the Doom series, the personal teleporters of Alien captains in Duke Nukem 3D)
Just plain diversity, especially common in regards to enemies meant to be more or less regular human beings. This in order to avoid the effect of feeling that the enemies faced are the same individual cloned countless times, usually to the effect of creating the impression that such cloning rather took place on three to five different individuals instead.
In Turok 2, Blind Ones, Fireborns, and Troopers are skin swaps of Sentinels, Endtrails, and Mantid Soldiers, respectively. The former two are literal underground mooks.
In the original Halo trilogy, Elites and Grunts got different-colored armor based on their military rank (determined largely by Asskicking Equals Authority), while Jackals got different colored shields and some minor armor changes. There were also special classes, like Honor Guards and Elite Councilors, with more distinct (and often far more ornate) armor.
While Brute ranks in Halo 2 were indicated by whether they had a bit of armor and/or a flag on their back, Halo 3 dramatically revamped their appearance, with each higher rank having more noticeably elaborate armor, while sub-ranks (Minor, Major, Ultra) were represented by palette swaps. The highest class, Brute Chieftains, have red or gold-accented black armor and warbonnet-like helmets. Halo: Reach re-simplified the system by doing away with the sub-rank palette swaps.
Halo 3: ODST gave Drones a ranking system, that was unsurprisingly also differentiated by color. The game also featured the only appearance of gold-armored Hunters fighting alongside their standard blue-armored counterparts.
Halo Reach was the first game in the series where you could tell what rank and specialty every Covenant mook had even if you removed all the color, due to dramatically increased differentiation between their armor, which has carried over into subsequent games.
The later levels of Pathways Into Darkness feature Ghasts (aka Earthquake Zombies), Green Oozes(which are invincible, but avoid the player if he is poisoned), Venomous Skitters (which as their name implies, inflict poison status), and Greater Nightmares (who are armored and shoot homing projectiles).
In Borderlands, when you're done with your first playthrough and start on a second one, the enemies get more health and different names, but their models don't change. Played straight in the first exapansion and the final expansion, which featured zombified and "-trap'd" enemies. The zombies had noticably different AI, but the claptrap-ised enemies just had different skins and dialogue.
Hack and Slash
Diablo I and II were full of this. Every single enemy in the games, apart from quest specific bosses, came in various levels of strength denoted by colour and had otherwise identical sprites as others of its type. It's mentioned in the first game manual that this is because the Prime Evils, the leaders of the demons, would alter their servants forms to better deal with whatever threat they were facing at the time.
Diablo III will probably be the same. Trailers indicate that it will also allow monster subtypes to vary in size.
It all started with Pac-Man, where the color coding of ghosts let the designers get away with only having one enemy type — the colors indicated different AI strategies in how they pursued the heroic circle.
Many early arcade games did this, due to the hardware limitations of the day. Some examples include Berzerk, Missile Command, and Pengo.
Super Metroid had half a dozen different colours of Space Pirates, of increasing power. From the wimpy grey Pirates in Old Tourian to the nasty red variant in Maridia that required the plasma beam to harm. There were also a pair of gold Pirates that served as sub-bosses before Ridley's lair.
There's a lot of enemies like this in Metroid games. Nearly every zone has its own variation of the basic Geemer (which itsef comes in a few different colour in both the original game and Super Metroid), Sidehoppers and Desgeegas are the same enemy with different skin, Ripper IIs are faster moving Rippers...
Monster Tale applies it not only to the enemies but also to one of the heroes; Ellie's partner Chomp has three basic body types (Child, Teenager, and Adult), and all of Chomp's various forms are variations of those - one may be the basic form plus wings, another plus a tail, with spikes, with just one eye, etc.
The game will often subtly alter the mesh of different "species" of the same sort of enemy (the Tengu, the minotaurs, the Nightfall insects, and so on).
Guild Wars Eye of the North has plenty of enemies recycled from the three previous campaigns, but the most noticeable example is re-using a species of monsters called "Mandragors". These insect/plant hybrids are found in the deserts of Nightfall, burrowing under the sand. In Eye of the North, identical monsters with the same name live in cold climate and burrow under snow, without as much as a Lampshade Hanging to explain it. See also: the frogmen, though this is lampshaded by the fact that each color appears to designate a different tribe. This doesn't stop them from being modified versions of the Heket from Nightfall, though.
Phantasy Star Online does this in Episode I. The Forest enemy type Boomas, the Caves enemy type Sharks, and the Ruins enemy type Dimenians all have the same basic "skeleton" and body structure, and the same attack animations. Later episodes seem to avert this, possibly because they were designed for more powerful hardware than the Dreamcast original.
World of Warcraft: Minor cosmetic variations on enemies are the rule, rather than the exception, due to the sheer size of the game:
Nearly every zone has some version of a wolf or boar to kill.
As humanoids go, nearly every zone has some version of a gnoll or murloc. They are just everywhere!
Terokkarantula is tougher than the smaller spiders nearby, as would be expected by it's named nature and elite status, but a player who hasn't been there before is probably not expecting a spider that's larger than a house.
In later expansions, the developers go through a good deal of work to create a few "unique" creatures for each expansion (especially the alien planet Outland). Still, you're unlikely to hit a zone that doesn't have at least two or three models you've seen before.
The creeps in Warcraft III are often subtle variations of each other, with the creep's size being a general indicator of its strength in comparison to others like it.
They have even hung a lampshade on the practice with a quest in Evendim, in which you are sent out for your umpteenth "kill me some boars" quest. It doesn't actually tell you to kill the boars, just look for some. This is important in that there are no boars in Evendim.
Which was further lampshaded in a later introduced dungeon in Evendim, where you actually can encounter boars. If you kill one, a quest starter item will drop giving you the quest to finally bring the original questgiver his boarmeat.
EverQuest used this extensively. It may have been possible to fight a "variety" of Skeletons — sharing one model and possibly one texture (with Palette Swaps) — all the way from level 1 to max level.
Everquest II has some sort of skeleton or zombie in almost every zone.
Prior to the first update in RF Online there were a fair number of reskinned creatures, although the story behind them made more sense. When the new "Episode" addons were introduced, many of the creatures were recolored and resized for the newer areas. In fact even the players became this, as equipment above level 50 would be a reskin of an earlier piece of equipment. The newest update migated this somewhat with Elfland, where the reskinned enemies were few and far between, and a lot of new models were made for it. Too bad you wont get a chanceto enjoy it.
Mabinogi is a particularly egregious user of this trope. There are a very limited number of enemy types; and they tend to get recycled constantly. The most blatant example are mongooses in Iria. They exist in nearly every part of the Maiz Prairie region, and are indistinguishable except by tail colour (even their names reflect this), with each colour indicating a different difficulty level.
Ragnarok Online has a few of these: (Archer/Soldier/Pirate) Skeletons, Kobolds/Goblins and to some extent Orcs, the Poring family and some others. There are also lots of monsters that have just one stronger palette-swapped version, and a few families of monsters that have the same name and behaviour, but different colors and attacks/stats/elements appearing in the same areas.
Likewise, in Super Mario Bros., red-shelled Koopas were implied to be "more powerful", at least in that they had enough sense to not stroll off of cliffs like their green counterparts.
This led to the Green Koopas marching in straight lines unless they came upon some sort of block or another enemy creature (like a Goomba), which would make them turn around. (They also tended to turn around if they walked into you, which was easiest to see when the action froze as Mario fell off the screen.) Red Koopas behaved the same way, except that they also turned around when they came upon a cliff (instead of just walking off the edge like the green ones.)note Granted, you wouldn't be laughing if a Green Koopa fell on your head, but... yeah.
There was a much greater difference between the Green and Red Paratroopas. Green ones tended to hop along in a straight line (leading to major headaches as you were forced to decide whether to try to dash beneath them or hop over them, and more often than not wound up in the wrong place at the wrong time); red ones just flew back and forth, "patrolling" a specific area without changing elevation. Sometimes the red ones instead flew up and down without any horizontal movement, and occasionally the green ones did that too. In any event, once you stomped on a Paratroopa and knocked its wings off, it would revert to the AI of its ground-bound counterpart (not that you'd notice if said former Paratroopa fell into a Bottomless Pit).
In Super Mario Bros. 2, Shy Guys came in pink and red, with pink being the marginally smarter. However, Snifits came in a rainbow of colors, each with different behavior. And yet there was only ever one red Snifit.
The second version of Mouser is a different color, takes twice as many hits to kill, and throws bombs with greater frequency. The original Doki Doki Panic included an even tougher albino Mouser, which was replaced by Clawgrip in SMB 2. There are also three versions of Birdo; the first is pink and spits eggs, the second is greenish and spits fire, and the third is reddish and spits both eggs and fire.
The Paper Mario games are prone to this in the later areas. Although each chapter tends to have its own set of themed enemies, a few will return as recolours. For example, there's normal reddish-brown goombas, a pink and blue pair of bigger "goomba bros", a king goomba, dark blue cave-dwelling gloombas, and greenish hyper goombas. The pit of trials that appears in each game lives off this trope.
Super Mario World has four colors of Koopas. Yellow drops from ledges like green, moves faster, drops a coin when taken out of its shell, and can jump into a shell to make it into an "invincible" flashing shell (or "disco shell"). Blue doesn't fall from ledges, and recovers much faster than the others after getting knocked out of their shells (they're also much thicker, implying they're stronger) and usually kick the shell away instead of reentering it.
It also affects the powers Yoshi gets from them: Green does nothing, red gives him a one-time flame attack, yellow makes him damage nearby enemies upon touching the ground, and blue lets him fly. Flashing shells give all powers at once.
For that matter, Yoshi himself: A green Yoshi eating a shell only gets the shell's power, but a non-green Yoshi also gets the power from his own color (so a blue Yoshi can fly with any shell).
Wario World does this a lot. There's a few unique enemies, but generally there's about four or five standard enemy types, and each world just changes the theme of them. You've got Magons, which then get reused as Skeletal Magons, Clowns, Snowmen, Wolves, Puppet Magons and Mummy Magons. You've got Cractyls which come as Bone Cractyls, Pigeons, Snow Bombers, Hawks, Masked Crows and Mummy Hawks. And the same for another four or so types of enemies.
Played straight in Wonder Boy III The Dragons Trap. Red foes are weak, green are stronger, and blue are the strongest, often with the blue variants being given surprisingly powerful attacks after the player had gotten used to fighting the weaker variants.
Several of the bosses in Wonder Boy In Monster Land do this, such as the Red Knight/Blue Knight/Silver Knight, the Grim Reaper / God of Poverty(steals your Gold), and the Giant Kong/Snow Kong(spawns mini-Snow Kongs instead of throwing boulders).
In Purple, basic mooks like slimes, bats and cannons get Palette Swapped at least three times, each with a slightly different behaviour.
In the first Donkey Kong Country game, Krusha came in two varieties. The first kind was blue with green camo and was only beatable by either of Donkey Kong's main attacks or a barrel (Diddy Kong's attacks were laughed off). The second kind only appeared once in the SNES version, in the very last level before King K. Rool. This version was grey with purple camo. The only thing that could beat him was a barrel, making him the strongest of the Kremlings.
The Super Hulk or Super Mech from Descent is a red version of the Medium Hulk that is much tougher and armed with homing missiles. The Fusion Hulk is a scaled down version of the first boss armed with a Fusion Cannon. In the second game, the Spawn is a green version of the Red Hornet, and the Tiger or Red Fatty Jr. uses the same model as the first boss, although it is smaller and has completely different weapons.
Bug! You fight snail enemies in Insectia Scene 3, each of which were very slow and took three hits to die. And then when you get to Splot, you see them again. Except that they take nine hits, and move twice as fast. And when they see Bug, they take out freaking MACHINE GUNS from their shells and fire at him!
The Stage 6 and 7 bosses in Blaster Master, which are otherwise palette swaps of the Stage 2 and 4 bosses, have different, more powerful attacks than their predecessors.
Pikmin uses this with common enemies as well as the titular creatures.
Pikmin themselves come in red, blue, and yellow initially, later adding purple, white, rock, and winged, all of these having their own sets of characteristics and combat abilities.
Bulborbs, the game's most iconic enemies, come in red (known as spotty in the originals), orange, snowy, and as the much more dangerous Bulbears, each having its own dwarf variety.
The cave-dwelling Dweevils in Pikmin 2 are red (fiery), blue (caustic), yellow (anode), purple (munge), and black with a bomb on its back (volatile).
Roguelike games employ this device to a fare-thee-well, since all of the monsters are represented by ASCII symbols, color coding is often the only easy way to tell them apart. Of course, sometimes two monsters have to share the same letter and color. Which leads many players to use alternate graphical "tile sets" which offer more information. Whether or not you should do this is one of the major fault-lines in NetHack fan circles.
It's interesting that this trope still appears in roguelikes not just in that the monsters look the same but in that there are different variations of the same monster, even though in those there is NO work required in generating sprites for new monsters, so the imagination is the only limit. Given that Angband has close to 1000 unique monster types, the reason for this happening in such a game is probably more that the designers started running out of ideas rather than not being able to animate distinct monsters.
Dwarf Fortress inherits this particular version, including the tileset option. For example, color is usually the only way to differentiate between rocks of different ores (which can be very important when you need to smelt metals) without looking at them specifically.
In NetHack, this can also lead to Yet Another Stupid Death, in ways both obvious and surprising. Not only might the player not distinguish between a dwarf lord and a mind flayer, in some contexts the game itself doesn't distinguish between them. Ooh a blessed scroll of genocide! You'd better cap the mind flayers, having to remap levels is a bitch. What's that? You were playing as a dwarf? Congratulations: you have succumbed to death by palette swap. (Blessed scrolls genocide a class of monsters, in this case h, Humanoid)
The sprite based Roguelike, Dungeons of Dredmor, has an absolute ton of these sorts of monsters. Most have different effects - Diggles are just annoying, but Sickly Diggles debuff you and Diggle Commandos are invisible.
Morrowind had a considerable lack of diversity amongst its native fauna, resorting heavily to underground monkeyism to create a wider range of enemies. This was somewhat justifiable in the sense that most of the game took place on a single island, and travelling northwest to the island of Solsthheim introduced you to a set of entirely new creatures.
Super Mario RPG has this with a LOT of enemy types. Sometimes in quick succession.
In the Dragon Quest/Warrior series, one of the first common enemies you encounter is the blue slime. Then, you meet the slightly stronger red slime. And as the series progressed, the Heal Slime. Then the Metal Slime. Then the King Slime. Then the Metal King Slime. Then the Mons game showed up, and you got Treeslime, and Wingslime, and Halo Slime, and... All the way up to the exceptionally powerful, Akira Toriyama-inspired (and designed!) Nigh Invulnerable Platinum King Jewel. It also featured color-coded dragons.
The majority of monsters in the series have at least one recolor, including regular bosses. In fact, the secret boss of Dragon Quest 6, arguably the single most glorified, powerful antagonist in the franchise, is a palette swap of an earlier boss. It's easier to count the few enemies with unique sprites.
Final Fantasy uses this in most incarnations, especially with the Elemental Rock-Paper-Scissors aspect: the blue monster casts water spells and is weak against thunder, the white monster casts ice spells and is weak against fire, etc. Final Fantasy X made some extra use of this, as a side quest rewarded players for capturing entire "species" of monsters. It was especially common in the earlier, sprite-based games due to Palette Swaps.
Final Fantasy XI has tons of instances of monsters that look exactly the same, only stronger and with a different name, including several Notorious Monsters.
And when monsters of the same species don't look exactly the same, they are palette swaps. In some cases this is justified. For example, rabbit type enemies have different fur color in different climates. It gets a little harder to justify with the Wings of the Goddess expansion, where forest tigers from 20 years ago are neon orange for no apparent reason.
Final Fantasy XII manages to subvert the spirit of this trope without subverting the letter of it, by making slight differences in the meshes of any given group of monsters (i.e. some toads have claws, others have webbed feet).
More prominent example: the Hellhound has a large horn that the Desert Wolf does not.
Final Fantasy XII also justified it to some extent, as many of the monsters who share a given sprite are in fact related to one another, as the Bestiary implies.
Final Fantasy XII is a good example of how to do palette swap enemies, with different species of monster varying by environment: their primary diet, the climate, and some being mutations caused by Mist.
Crisis Core: Final Fantasy VII took this to ridiculous extremes. If a monster got a special white coloration with feathers, & a pattern of a certain character's face, it became a 'copy' of said character. Despite essentially being the same monster, it was implied to be much more powerful as a result.
Worse yet, after a certain point, they didn't even bother to change the enemy colors. This left you with about 200 missions of fighting the same, boring avatars.
Both Final Fantasy VIII and Final Fantasy IX avert this trope, where (almost) every monster type in the game has a unique mesh, animation, and sound effect. The only exceptions (for VIII, anyway) are for human and humanoid enemies, plus Elvoret and Elnoyle.
IX has a few palette swaps; the ten Fairy Battles, and the Crystal versions of the Four Fiends.
Gameboy games had the lack of memory needed to justify Underground Monkeys, but, being monochrome, had no way of switching palettes. Generally, this led to enemies either never improving or simply gaining more hit points, though turn-based RPGs (Such as the SaGa/Final Fantasy Legend series) were able to circumvent this by... giving the sprites new names. (Say, isn't that Master Dragon the exact same size as the Baby Dragon from the start of the game?)
Interestingly, this hardware limitation led to an inversion in Metroid 2. In the original, getting Samus's Varia armor made the sprite change color, which wasn't possible on the Gameboy. To compensate, gaining the upgrade gave the Varia suit the huge shoulders that are now the character's trademark look.
Metroid: Zero Mission, which is a remake of Metroid 1, lacks the round shoulders on the Varia Suit. However, near the end of the game, Samus's Power Suit is destroyed, and she obtains a new one. The new one has the large shoulders. The end of Zero Mission takes place in the immediate aftermath of the ending of Metroid 1, and as such explains how Samus came to possess the Metroid 2-style Varia suit.
In the Fallout games, a common practice was to have different individuals represented by a sprite of the same person (usually the male and female sprite used for the Player Character, no less) stuck in a different suit of armour or clothing. The second game in the series contained a couple of self-conscious Lampshade Hanging jokes on this theme, including the henchman of a crime boss confiding that he suspects there must have been a big cloning accident at some point in the past, and an Easter Egg location in which a pair of sprites originally intended to be player characters but retooled to only fill NPC duties lament over their fate.
Fallout 3 is a more modern 3D game and as such gives every human character different appearances. And while all Feral Ghouls look mostly the same, the different species of Ghoul are easily identifiable. Ferals are the standard, Reavers from the Broken Steel DLC get a different face and abilities and Glowing Ones... well, glow.
Broken Steel also adds Hellfire Troopers, which are Enclave troops with high-impact fire-resistant armor and heavier weapons, and Albino Radscorpions, albinistic versions of Giant Radscorpions that are much faster and tougher, and regenerate HP in sunlight.
Fire Ants look like normal Giant Ants but also spit fire. Like real ants, they both have several ranks with varying appearances and statistics.
Mirelurk Hunters are bigger, tougher, and have a different model than base Mirelurks, Nukalurks are a glowing blue version of Hunters found in the Nuka-Cola plant, and Mirelurk Kings are a completely different species, being mutated turtles rather than horseshoe crabs.
The Mister Gutsy robot is an upgraded version of Mister Handy with better armor and a plasma cannon in addition to the standard flamethrower.
Normal Super Mutants, Overlords(Broken Steel) and Behemoths are very easy to tell apart and are fought with different strategies; unfortunately the Brutes and Masters only have slight armor and HP varieties from the standard ones. Overlords use the same model as Behemoths but are smaller and wield tri-beam laser rifles or gatling lasers, whereas Behemoths exclusively wield fire hydrant polearms.
The Point Lookout expansion adds swamp versions of Feral Ghouls, Mirelurks, and Mirelurk Kings, as well as the Robobrain Sentries that guard the Final Dungeon, which really are just palette swaps with slightly different abilities. Except for the Swamplurk Kings/Queens, which have 100 HP more than a Deathclaw and a new acid attack that deals 100 HP damage plus poison health drain.
Likewise for Maintenance Protectrons (lack the US Army livery) and Nuka-Cola Security Protectrons (painted with the Nuka Cola logo) in the base game.
In Fallout: New Vegas and its add-ons, the Spore Carriers are reskins of the Trogs from the Fallout 3 DLC The Pitt, with an added Action Bomb capability. The Trog model is also reused for the Tunnelers in Lonesome Road, some of which can knock down or poison the player character.
Lakelurks are renamed and slightly recolored Mirelurk Kings.
Old World Blues introduces the Mr. Orderly, a medical version of Mr. Gutsy, Construction Drones (repaints of Protectrons), and Berserk Securitrons.
Also in OWB, Police and Military Cyberdogs are mostly identical, except for the latter's sonic emitter attack.
The Feral Ghouls in Vault 34 wear vault jumpsuits or armor and are named according to their human roles, but statwise are identical to normal Ferals or Glowing Ones, except for the Overseer, who is a boss version of the Reaver type with stats similar to FO 3's Reavers (minus the radioactive gore attack). Similarly, Camp Searchlight is inhabited by Feral Ghoul-ified NCR Troopers, which wear trooper armor and attack with knives or other melee weapons.
Golden Geckos are upgraded yellow versions of the standard Geckos, Fire Geckos are spinier and breathe fire, and Green Geckos from Honest Hearts are twice as large and spit acid.
In addition to the ranks from Fallout 3, Super Mutants have a blue-skinned variant known as Nightkin, who are equipped with Invisibility Cloaks and different apparel and weaponry than the normal Mutants.
Deathclaws have several new variations, including Mothers, who have blue skin and swept-back horns, and Alpha Males, who have dark skin and elongated horns. Both are notably taller, faster, and stronger than the vanilla Deathclaws.
Bark Scorpions use the same model as Radscorpions, but are smaller, brown in color, and have more potentpoison.
In Brave Story: New Traveler, the exact same enemy can come in multiple different colors, so the difference between genuinely different enemies is at least slightly greater, with a few exceptions. This editor isn't quite sure how to label this.
This was part of the game's world design. Monsters were designed to be "organic" in that one given monster species would have biodiversity. The player could encounter three mobs in the same battle, and each would have slightly different stats. In general, colour had less to do with power than size, with larger mobs being significantly stronger.
Persona 3 is another good example of this trope—practically every enemy inside Tartarus, the game's sole real dungeon, uses one of a select number of sprites, and most sprite-sharers are vulnerable to the same kinds of tactics (if not necessarily always sharing elemental weaknesses).
Not only that, but every boss not important to the story is simply a giant version of a normal enemy, a practice that would carry over to Persona 4.
Persona 2 also has a small handful of these, thanks to palette swaps and the occasional replacement part in monster sprites (several Chariot Arcana demons the most obvious of the latter, using the same giant brute body with different heads and colors).
The Monster Hunter series uses this repeatedly. A prime example is Yian Kut-Ku, one of the easiest wyverns in the game, which appears later on as the Blue Kut-Ku, a stronger (and blue) version identical in every other way. Not only does the game do this with the enemies, but the armour then made from the enemies looks exactly the same apart from a colour swap to fit with the wyverns' colour.
Kut-Ku is one of the few cases where the alternate version doesn't have an appreciable difference in elemental weaknesses, body part strength, moveset or move frequency. For example, a green Rathian's head is its weakest part, and it will charge at you a lot, with occasional backflips thrown in. Conversely, a gold Rathian's wings are its weakest part, and it backflips all the time (which ironically makes them easier to deal with).
Some other examples of monster counterparts behave exactly the same but have very different appearances. For example, the Aptonoth looks like an amalgamation of different species of dinosaurs, while the Tundra-inhabiting Popo looks like a very short, trunk-less woolly mammoth. Both species behave pretty much identically, and they both fill the role of the harmless herbivore that gets eaten by everything else.
Grandia II starts doing this about halfway through the game.
Pokémon does not perform "Palette Swaps" of its monsters aside from rare "shiny" Pokémon which are the exact same species. The closest thing to recycled enemies might be evolved forms of previously encountered Pokémon, or the same Pokémon at higher levels. This lack of palette-swapping monsters makes sense when you consider the whole premise of the game is built on having a wide selection of Pokémon to customize your team with. There are many Pokemon based on similar animals who are mostly differentiated by the location they live in though. Take Pachirisu, an electric squirrel of the Sinnoh region and Emolga, an electric flying squirrel of the Unova region. Pikachu, a rare electric mouse that lives in forests and Maril, a rare river dwelling mouse. Also, the same species of animal may be represented more than once in the same area such as Pansage, Pansear and Panpour who are all forest monkeys but each of a different elemental type.
Played completely straight with some of the Trainers - While there are no palette swaps, and the in-battle sprites of the Trainer types are all unique, often several types share the same overworld sprite, so that in Pokémon Diamond and Pearl, for example, Ace Trainers and Rangers look the same until you fight them (and the sprite is clearly that of the Ace Trainers). The most egregious example is perhaps the Rich Boys, Psychics, and PIs/Gamblers. PIs/Gamblers wear a fedora and a trenchcoat in battle, while Psychics wear a purple jumpsuit with greenish hair; the overworld sprite used for all three clearly has the Rich Boy's dark purple hair and white suit.
Although the reasoning behind PIs having Rich Boy's look could be the fact that they're supposed to be incognito. Psychics, however, have no excuse.
An In-Universe example would be Shellos and Gastrodon, which are only cosmetically different on the West and East sides of the continent. The justification was divergent evolution (that is, the real kind) after being separated by Mt. Coronet.
Also played straight with the cry sounds. There were only about 30 cry sounds in the original game, and while some were sped up or slowed down, some cries, even by unrelated Pokémon, were identical. Charizard and Rhyhorn, Ditto and Poliwag... Caterpie and Goldeen technically have different cries, but you'd have to listen to them in succession to hear it. Less noticeable in Pokémon X and Y, where the revamping of the Pokémon cries meant that the cries of the original Gen I Pokémon went through Divergent Character Evolution. (but they still sound similar.)
Interestingly, this was played straight with Shinies in Pokémon Gold and Silver: unlike in later games, which have a "shiny flag" on the Mons, Pokémon with specific base stat totals were Shiny, which resulted in Shiny Pokémon having somewhat higher base stats.
Taken to a more extreme level by the Wizardry games, particularly VI and VII, wherein enemies were given graphics by type-all slimes use the same graphics, as do all demons, all bugs, etc, including non-hostile NPCs and bosses. Further complicating matters is that unless a party member has a high mythology skill, all you'll see attacking you is generic "birds" or "crawling wastes". Experienced players can usually determine what particular monster is attacking them by the area they're in or the attacks the monster uses. Occasionally leads to party kills when the player mistakes a very nasty enemy for an easy one.
Chrono Trigger includes the "Debugger" and "Debuggest" robot-bug enemies; "Rolies", "Polies", and "Rolypolies"; "Cave Apes" and "Goons"; and "Mutants" and "Metal Mutes", among others.
Golden Sun has this all over the place, although there are certain occasions where an enemy will be slightly powered up without changing the name/color.
There are even some unused variants of certain enemies hidden in the game's coding, complete with differing Palette Swaps and names.
The later sprite-based Might and Magic games, especially the seventh incarnation, played this trope hilariously straight. How do you tell the difference between a minotaur and a minotaur lord? The nastier version is almost identical - it's just a bit bigger. And bright purple.
Albion does this a little differently. The enemies are different on each continent, but come in a small variety. Stronger versions of certain creatures accompany larger packs. They don't even bother with creative names (Animal3)
The World Ends with You has four or five varieties of every monster type in the game. Including bright pink elephants. At least they have slightly differing attack patterns and (sometimes) vulnerabilities.
In MOTHER 1, the Lone Wolf, Silver Wolf, and Wolf are all the same sprite with different colors, and the Stray Dog is a wolf sprite colored brown with a chain around its neck.
The entire trilogy does this, although EarthBound and MOTHER 3 give the palette swaps goofy names. For instance, you have the 'Manly Fish' and his stronger swap, the 'Manly Fish's Brother'.
The Swedish parody RPG Playelf has this with ninjas - there are red ninjas, blue ninjas, black ninjas, white ninjas, etc - as well as "hurry up-ninjas" which appears when the players are dithering. But the most awesome ninja...
Present throught out the Tales Series. Monsters that are purely palette swaps are most common in the games that utilize sprite based graphics, while the 3D games usually change their model a little, as well.
Boktai has this in all its incarnations, though sometimes coloration is used as a hint to its elemental affinity. This is more egregious in Lunar Knights, where many enemies are colored solely by affinity - namely, the Ghouls, Vorns, Slimes, Hounds, and Chloroformin' come in different colors on this alone. The Slimes, strangely enough, are the only ones in this group that come in Sol flavor.
Ultima V contains literal monkeys found only underground — the Mongbats — but they resemble nothing else in the game.
Ultima III has multiple enemy types with the same colours where the only difference is the name - however, that's literally the only difference. No change in stats, health, damage dealth, weaknesses... Just Orcs, Goblins and Trolls, all exactly the same.
Done frequently by the Mario & Luigi games. Most of them are simply Palette Swaps with boosted stats that attack faster, but some such as the Dark Mechawful from Bowser's Inside Story diversify themselves from their vanilla cousins. Dream Team is particularly egregious, as many Dream World enemies are altered versions of Pi'illo Island enemies, and several other enemies get stronger R forms. Of particular note is the Capnap, which also comes in Dreamcap, Dreamcap Captain, Dreamcap R, and Dreamcap Captain R varieties.
Evil Islands: You'll find several types of wolves, boars, toads, tigers, trolls... that look the same except for color or size.
Shining Force uses this in all of its games and it is jarring. The original, for example, had regular Bats, and their aquatic cousins, Sea Bats.
Non-Video Game Examples:
Digimon as a whole loves this. The anime itself has recolors who are merely a different attribute (such as say, Black Rapidmon, who is a Virus-type counterpart to Rapidmon), different element (such as Yukidarumon and Tsuchidarumon, snow and ground respectively), or just a recolor for the sake of being a recolor.
The Digimon games add to this by not only having the original recolors included, but several entirely recolored evolution lines, Rookie to Mega, in Vaccine, Data and Virus flavors.
Taken to an absurd extreme with Soulmon, who is a Bakemon with a wizard's hat. That's the ONLY difference.
Oh? No mention of Nisedrimogemon, the Digimon that Bandai took a regular Drimogemon from and merely slapped a swirly mustache on?
Color-coded dragons predate most video games, as they appeared in the Tabletop GamesDungeons & Dragons, which was first published in 1974. Evil ("Chromatic") dragons have scales of a particular solid color reflecting their place in the Elemental Rock-Paper-Scissors spectrum, and good ("Metallic") dragons have scales of precious metal. Interestingly, though, these aren't "palette swaps"; as it is possible to readily identify different species of dragons in greyscale artwork (for example, white dragons have a peculiar vertical crest on their head, while silver dragons have backward-pointing horns and a ribbed frill along their necks).
On one occasion, the color-coding is used as the basis of a truly heartbreaking Monster Is a Mommy story, when a noble silver dragon is born with albinism, and is hunted down and killed by an adventurer who thinks it's a white dragon.
One adventure featured a similar story with an albino red dragon, causing the party to prepare to fight it in the least effective way possible.
A particularly infamous example is the Rothe, which is a semi-literal Underground Monkeyof a bison.
The Elves of D&D come in high, wood, sea, grey, wild and several other varieties.
The third edition of Dungeons & Dragons features templates, giving uncreative GMs the opportunity to color-code any monster into a water monster, a fire monster, slime monster, etc.
Although one 3rd edition Dragon article about creating monsters included "But this one's blue!" in a list of how not to do it.
Fourth Edition includes at least one extra variety of every monster in its Monster Manual entry. Many of these fall into Underground Monkey status, being simply higher level versions from a different environment-normally, a different plane.
While discouraged due to the WYSIWYG rule of the game, Warhammer 40,000 still has shades of this. Space Marines have more rules than the rest of the other playable factions combined, and the only way to tell them apart is by their armor and what kind of bling they have (Robed and Green, it's probably Dark Angels. Spikey and/or evil looking, probably Chaos. Red and Blood drops, Blood Angels. Knightly helmets and book emblems, Grey Knights. Swords and seals, Black Templars, etc...). Even within a single army, the difference between an elite squad of veterans armed with modified boltguns and a simple tactical squad is sometimes literally a differently painted shoulderpad. Averted with the other races, where each type of trooper generally gets their own model.
There's now specific models for veteran Space Marines, with custom boltguns that look a bit different and shoulder pads that have embossed icons for non-codex chapters. Of course, these models are more expensive, so some players stick with the old palette swap method, especially since some veteran models are non-customizable and don't have optimal wargear.
Other toys use a similar system, often called redecos (when identical molds are used but the color of plastic is altered) or retools (when most parts stay basically the same but are altered to include, for example, new accessories; this can also include a redeco). For example, in one series of Transformers, Stormcloud is a redeco of Powerglide, while Sideswipe is a retool of Sunstreaker.
This even leads to recolors in characters in the cartoon—for example, Thundercracker and Skywarp were repaints of Starscream. This became very confusing when someone accidentally colored two Starscreams.
The most annoying use of this in recent series has been making Galvatron a redeco of Megatron.
For most of its run, BIONICLE sets were just recolors of each other with slight differences in assembly and parts. Sometimes, the only difference would be their masks and their tools/weapons. Then, the Mahri Nui saga came along in 2007, and the Barraki were released, each looking very different from each other. Since then, the sets have been largely Averted this trope.
Played with in the larger sets. While each large set was in of itself different from other large sets, each early large set could produce two near-identical creatures. Justified in that back then Bionicle toys were more akin to Rock'em Sock'em robots, and the kids were expected to play against eachother so the toys had to be identical to make it fair. Newer large sets averts this since they lost the Playability of the older sets in lieu of posability.
As mentioned in the trope description, the Trope Namer is RPG World, and was originally a Lampshade Hanging by showing a number of "Underground (animal not usually found in caves)" enemies in succession.
Parodied in the webcomic Order of the Stick: in one comic, a paladin discovers the titular party has killed a dragon. She then accuses them of possibly killing a creature of benevolence and wisdom, and asks why they thought it deserved death, to which Roy Greenhilt replies, "Erm... its scales weren't shiny?" which placates the paladin. Elan then breaks the Fourth Wall by winking at the reader and saying, "Dragons - now Colour-Coded For Your Convenience!"
Ironically, the comic does this itself with goblins/hobgoblins/ghouls.
Vaarsuvius rants about the use of this trope to populate the Underdark in Snips, Snails, and Dragon Tales.
Vaarsuvius: It is exactly the same as the upper world, only dimmer! JUST STAY AT HOME AND PUT ON SUNGLASSES!
Lampshaded in the webcomic Adventurers!, where the Ice Dragon, boss of the Ice Cave, paints himself red and calls himself a Fire Dragon to fool adventurers.
Truth in Television: Members of the same biological genus are usually similar enough that the main differences are color and location. Things like size, basic body shape, and diet are all almost identical.