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.

Examples:

Arcade Game

• 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.

Eastern RPG

• 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.
• The Mole Playing Rough in Earthbound. 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.
• 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 Gao Gai Gar where the monsters were transformed humans and their looks based on their personality and the environment.
• Sailor Moon Another Story uses as Random Encounters monsters that were originally monsters of the week in the show. The monsters in the show were either transformed people or transformed objects, so like in the Super Robot Wars example it doesn't make sense that there would be armies of them.

First Person Shooter

• 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.

Massively Multiplayer Online Role Playing Game

• World Of Warcraft: Rockflayers are creatures natives of Draenor, but somehow, they can also be found in Deepholme, the Elemental Plane of another world.

Platform Game

• Arguably the infamous Metools from the Mega Man games could qualify. 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.

Real Time Strategy

• 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.

Survival Horror

• 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.
• Silent Hill 2: Pyramid Head is explicitly made from part of James' damaged psyche. Doesn't stop him from showing up in later games that have absolutely nothing to do with James.
  • Also in Silent Hill 2, the Abstract Daddy is sprung from Angela's psyche. When it reappears as a Degraded Boss, Angela is nowhere to be found.

Third Person Shooter

• 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.

Western RPG

• 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.