Possible names: Mook Recycling.
When you encounter a new type of enemy in cinematic video games, it is often introduced in some sort of context that justifies its existence. After that point however, designers may continue to use that type of enemy in regular rotation even once you've entered places where its presence doesn't make sense, or you may continue to encounter that enemy in quantities far beyond what its origin story would seem to explain.
Boss Rush and Degraded Boss are often cases of this.
Compare Giant Space Flea from Nowhere, that's when they come from nowhere with no context. This is the enemy equivalent of a Voodoo Shark, where the explanation doesn't serve to explain how there are so many of there things all over the place. Contrast Underground Monkey for when enemies are recycled, but come in a slight variation for a given environment.
Games that do not attempt to provide fictional justification for their enemy designs are immune to this trope.
Luigi's Mansion: Dark Moon has a few in the same game as well. Like the 'super'/'strong' ghosts, which are initially encountered as experiments in the Secret Mine, but then just show up all over the place in the last mansion for no real reason. As well as on every possible floor of the Thrill Scare Scraper. There's also one that just happens to appear in the mansion before the one you find these experimental ghosts in 'for the first time'.
Or the Carnivorous Plants that initially show up in the gardening themed Haunted Towers with other plant enemies, and then somehow show up in the final mansion's kitchen in a cupboard (along with the more reasonable Jungle Exhibit).
In the first level of Ninja Gaiden on Xbox, Murai sends his own, novice mooks at you in normal mode. From hard mode on, though, the same level makes you fight Black Spider Ninja... which are supposed to be enemies of Murai and have no reason to obey him whatsoever. Similarly, in hard mode you will often fight fiends and Black Spider Ninja altogether, even though these fiends are supposed to fight the Ninja, not help them.
In Final Fantasy VI, there's the Veldt, which contains every enemy you previously encountered (to allow obtaining their skills in a specific form of Mega Manning done by the characer Gau). This includes soldiers, elite soldiers, and enemies said by the story to be already extinct. Some bosses also appear in the Veldt, such as the Senior Behemoth and the White/Holy Dragon.
The Mole Playing Rough. They first appear in the Lilliput Steps, an underground cave, where they're a decent foe (oh, and the boss there is a giant mole, so there's a mole theme in that cave). However, for same reason, the designers put some specific points (to be exact, in the Dusty Dunes Desert, Summers Beach and the Deep Darkness) around the game where one of them always spawns if you walk around there. Not only they're incredibly weak by that point, but they're places where you wouldn't expect to find a mole.
Another famous example is the Mad Ducks (an enemy encountered back in Winters) behind the store in Saturn Valley. Even stranger is the fact that they are in an area that you can't reach unless you exploit a glitch.
Mad Ducks also appear in the underground tunnels in Dusty Dunes Desert. They're incredibly weak enemies at that point (to the point that, to be able to provide a bit of challenge, they spawn there in absurdly high numbers), and it also makes one wonder what a duck is doing in a desert cave.
Talah Rama's cave in Dusty Dunes Desert is for some reason populated by enemies by found back in the Milky Well Cave. Not only are they weak at that point, they'll also run away from you if you've already defeated Trillionage Sprout (which most likely you'll have done by that point).
The area between Threed and Dusty Dunes Desert will sometimes have New Age Retro Hippies, enemies encountered back in Twoson. They're also on a high ledge, which your party cannot reach. note This example in particular happens because that area is actually placed in the game files near the tunnel entrance in Twoson. There's a enemy spawning point located on the Twoson tunnel entrance, which is also active even if you're on the Threed side. Sometimes, an enemy can spawn there as if you were in Twoson. However, you wouldn't know that unless you're inspecting the game ROM.
The Dungeon Man is full of these. His first floor contains enemies that were in the Fourside Department Store. The dead ends on his second floor contains enemies from Moonside, an area you can only enter once. His third floor contains a "monster zoo," which invokes this trope.
Super Robot Wars does this for some Monster of the Week series: Formerly one-off enemies suddenly appear in droves. Can be jarring in cases where GaoGaiGar where the monsters were transformed humans and their looks based on their personality and the environment.
Subverted in the Rainbow Shell quest in Chrono Trigger. It seems really strange to be fighting Naga-ettes and Gnashers, enemies that appeared in the 600 A.D. Cathedral at the very beginning of the game. That is until you fight the boss, Yakra XIII, who is the descendant of the boss of the Cathedral.
A lesser-known example is the electricity-shooting skeletons in Niel Manke's Half-Life mod "They Hunger". The first one is encountered in a laboratory where it is brought to life by being electrocuted, which does much to explain why it attacks by zapping you with bolts of lightning. But then you continue to encounter electric skeletons for the rest of the game, all over the place, even though it's impossible to think that every instance of this enemy could have been created in the same way.
In Half-Life 2, Headcrabs appear at first mostly in places where the Combine have specifically used them as a biological weapon, and they're usually accompanied by the remains of their artillery shell cages. After the Ravenholm level however, they become more-or-less just Goddamned Bats ready to pop out of any vaguely abandoned area.
When you add up all the bodies, the extensve bloodstains, the skeletons, the zombies and the Lost Souls you fight in Doom 3, there seem to outnumber the human population in an area quite a bit.
Averted in Metro2033, Librarians remain in the library, Nosalises don't leave the tunnels and Dark Ones aren't encountered as normal gameplay enemies at all.
The infamous Metools from the Mega Man games. In the original Mega Man game the little hard hat guys appeared only on Guts Man's stage, which had the look of a quarry/construction site. (Guts Man himself appears to wear a hard hat) however they have since appeared in every Mega Man game on multiple levels and in massive numbers to the point that they are the most common enemy encountered.
In most strategy games, upgrading a unit will cause all units of that type to instantly improve. In Age of Empires, outposts not visited in thousands of in-game years can suddenly replace their clubs with swords and their loincloths for suits of plate armour.
Warcraft III: In one mission, Malfurion and Tyrande discover spiders that have grown to gigantic size when they came into contact with demonic corruption. However, there are many more giant spiders in the world, both in this game and in WoW, spiders that have never met any demons.
In Brutal Legend, it's possible to miss two Side Quests available during the campaign against General Lionwhyte and his glamed-up army. Since these quests assume you're still fighting him, coming back to complete them has you fighting Hair Metal units past a point in the story that they've all been wiped out.
Dead Rising famously gives you the Zombie Genocider achievement for killing a number of zombies equal to the entire population of Wilamette. Even accounting for out-of-town visitors and the infection spreading, it seems rather unlikely that that many zombies would wind up in the mall. And they still keep coming.
The babies in Dead Space. You first encounter them in a prosthetics lab with babies growing in tubes all over the walls. Nice and creepy. But even though this lab is only a single room with maybe 50 baby tubes total, from that point on zombie babies are ubiquitous all over the ship and you fight at least a hundred of them in the game.
Used as an actual plot point in Kid Icarus: Uprising, where enemies of various factions get copied in later chapters to fight alongside one another, despite them originally being hostile to each other.
Pit: So the Chaos Kin is copying Aurum enemies that are copies of the Forces of Nature.
Husks in Mass Effect 2 can fall into this, especially when you start wondering how many people there had to be on this single scientific expedition in order for there to be so many husks.
Fallout3 has Super Mutants, which appear far more numerous than their origins would suggest. Likewise, Raiders often seem t outnumber townspeople and wastelanders.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.