The boss in a video game, or on rare occasion VG series, who, after you defeat him/her/it, returns multiple times - but not as a boss, but as a regular enemy (sometimes more than one appearing at once). Sometimes, the boss you fought is the "strongest" of the monsters; sometimes you've attained a new weapon which is particularly effective against that boss, or just leveled up enough that you're able to take on several at a time. In some games, later enemies will be palette-swapped versions of the boss' sprite/model, and may actually be stronger than the original Boss form. This tends to be puzzling when the first one you face is said to be their ruler. Logic suggests that they should be the strongest of the lot, but when you face their higher-levelled brethren later on, one wonders why they aren't in charge.
Sometimes confused with Villain Decay. The two may overlap, however, if The Man Behind the Man pops up and reveals that there are multiple copies of his previously-unique underling. In narrative, this looks a lot like the Conservation of Ninjutsu. Sometimes results in Villain Forgot To Level Grind. Not to be confused with the Boss in Mook Clothing. Compare with Boss Rush where previous Bosses return as Bosses in rapid succession.
A common version of this is for the first Boss of the game to reappear as a Giant Mook throughout the rest of the game.
See Recurring Boss for examples where they don't get degraded.
The first The Legend Of Zelda game was rife with this. Dodongo, Manhandla, Gleeock, Digdogger, and Gohma all reappear in later dungeons as normal enemies. Dodongo (as noted above) even appears in threes later, Gleeock grows extra heads (from two in its first appearance up to four eventually, although the latter was for when it was reused as the boss of Level 8), and Digdogger splits into three after playing the Flute anywhere but its debut. That's just the first quest; they show up sooner and more often in the second one. In the case of Dodongo, in Ocarina of Time and Majora's Mask, Dodongos are run-of-the-mill nuisances. However, Ocarina of Time has King Dodongo as the boss version, operating much like the original Dodongo.
Ocarina of Time also had pairs of Lizalfos presented as Sub-Bosses in the second dungeon, while you play Link as a child. Adult Link can take them out in two hits with the Biggoron Sword, and by that point in the game young Link has enough equipment and hearts to turn them into mooks. Stalfos are another example, as they also act as minibosses in their first encounter, but you also encounter them as regular enemies later. Then they are used as minibosses again, with the added challenge of beating them both at once in a short time, lest they rise up again.
Nearly every miniboss in The Wind Waker is encountered as a regular enemy some time after their original appearance. For example, the large Moblins with spears are presented as inmune guardians Link must avoid confronting, then one is fought as a Mini-Boss in the fire dungeon. Afterwards, however, they're reduced largely to tough minions. This same role is later filled by the Darknuts, the first of which shows up as a sub-boss in the Tower of the Gods, but the very next section pits you up against about six of them and twelve Moblins (their later incarnations are more powerful, but you almost always fight at least two). Same case with the shielded Bokoblins, Mothulas and Stalfos.
In a meta sense, the Mothulas went from being a boss in A Link to the Past and Oracle of Seasons to a miniboss and later mook in The Wind Waker. Arrghus also went from an A Ltt P boss to a miniboss in Majora's Mask (called "Wart" outside of Japan). Gyorg was both played straight and subverted - In Majora's Mask it was a boss, in The Wind Waker a common mook, and in The Minish Cap went back to being a boss, then went back to mook status in Phantom Hourglass and Spirit Tracks.
Hardly unique among the Game Boy games. Many of the mini-bosses from early levels of The Legend of Zelda: Link's Awakening show up as regular enemies later in that same game. In some cases, there's a new weapon that makes it easier In other cases, not so much.
Cross-game inversion: Vire, a regular enemy from the original and Link's Awakening, shows up as an extremely annoying (both in gameplay and in taunting Link) sub-boss in both The Legend Of Zelda Oracle Games, in their respective sixth dungeons. Another cross-game example played straight: Facade, the boss of Link's Awakening's sixth dungeon, reappears as a miniboss of two dungeons in Oracle of Seasons.
An inversion in The Legend Of Zelda The Minish Cap, where Chus are common enemies, but one appears as a boss in one dungeon when you are the size of the Minish. But then played straight again three dungeons later, when you encounter another giant Chu as a miniboss. This one is stronger in that it can electrify itself, but your sword has been upgraded to match.
Twilight Princess examples: In the Forest Temple (the very first temple of the game), Link fights a giant carnivorous plant that swallowed an important key as a mini-boss encounter. Later, in the City in the Sky, there's one such plant growing in a room, reduced to minor nuisance status. The City in the Sky itself also features a fight with an Aeralfos, a winged reptilian creature that uses a sword, shield, and a suit of armor. The Cave of Ordeals features scores of such creatures. Zig-zagged with the Darknuts, however, which switch from miniboss (Temple of Time), to enemy (Cave of Ordeals), then miniboss (Hyrule Castle at the mid), and enemy yet again (Hyrule Castle at the end).
Two of the mini-bosses in The Legend Of Zelda Spirit Tracks actually appear again as regular enemies within their own dungeons (specifically, they're Snapper, the whip-wielding guy from the Ocean Temple, and Heatoise, the giant tortoise from the Fire Temple). In both cases, the dungeon item you get from the first battle makes the later ones much easier.
In Skyward Sword, Moldarach plays the trope straight (boss in the third dungeon, miniboss in the Shipyard), while Moldorms zigzag it. One appears as a sporadic, optional enemy in a grotto from the Fire Sanctuary, but the next one is fought as a miniboss later in the same dungeon. The ones found afterwards (one in a grotto during the Stealth-Based Mission in Eldin Volcano and another in the grotto of a certain island in the Sky) are regular enemies, but other two are minibosses in the final dungeon.
Happens twice in Onimusha. Reynaldo, who is set up as a mini-boss but is quickly revealed to be a really tough (and regenerating/self-duplicating) mook. Then there's Volchiman, who you must first fight as ninja girl Kaede, who is significantly weaker than main character Samonosuke; later on you may encounter two Volchiman at once. However, there's also Marcellus, whose prototype you face first. The real deal is a much, much more difficult opponent - probably more so than final boss Fortinbras.
Almost every single miniboss in Killer7 becomes this, since the miniboss battles are meant to introduce a new type of Heaven Smile for the next chapter. The first miniboss and the last two never show up, however, though the penultimate miniboss, the Timer Smile, is debatably a major boss, as there is no boss in its section after it.
One of the various mech bosses in Shadow Complex returns as a regular enemy at one point, but strangely still has his boss lifebar if you're near it. In the final boss fight you fight all of them.
Every enemy is introduced in The Haunted Mansion with a lot of fanfare, and you are put into a one-on-one match with them. As soon as you beat them, they begin showing up as regular enemies. Fortunately, Zeke's weapon gets powerful enough to accommodate for this.
The Hell Vanguard from Devil May Cry 3 returns in later parts of the game as a lifebar-less mook. However, all recurrences are as strong as the first one. Furthermore, on one occasion (Mission 17's Trial of the Warrior room), the Hell Vanguard "mook" taps into the latent "Devil Trigger" power and becomes even more powerful than the boss version. On the highest difficulty, all "mook" versions of the Hell Vanguard can potentially use the Devil Trigger power.
Devil May Cry 4 combines this trope with Cutscene Power to the Max. After defeating the frog demon Dagon, you are treated to a cutscene where Dante receives a new Devil Arm. Suddenly, the courtyard is full of Dagon's brothers. Dante then proceeds to use his new weapon to quickly annihilate every giant frog in sight.
Played with in the first game. Phantom, Griffon, and Nelo Angelo all are bosses you fight multiple times in game, and each actually gains new attacks and abilities in between fights. However, after you kill them, when fighting the boss Nightmare, it can absorb you and force you to fight a weaker illusionary form of one of the prior bosses.
A twist occurs in the first two Ninja Gaiden games for the NES: the Malice Four from the first Ninja Gaiden appears in Ninja Gaiden II as mooks who are literal clones of the originals created by Ashtar. The clones are the same size as Ryu and are killed with a single strike like other regular mooks.
Done both in-game and in-story in Ninja Gaiden II: The Dark Sword of Chaos. The basic Mooks you first fight are in fact clones of the first boss of the original game. Clones of the games subsequent three bosses also appear, usually in Giant Mook form, though Kelberos comes back in full Boss form.
The boss you face in the first chapter of seventh generation Ninja Gaiden II is Rasetsu, a large, literal Demonic spider/Human hybrid who stands about 8 feet tall with 4 large bladed appendages protruding from his back. You also encounter another Spider Ninja Fiend in the second chapter, backed up by spider ninja Mooks. In the last chapters you end up slaying a couple dozen of them; sometimes in pairs. They are still as strong as the first encounter, but by then you're a lot stronger.
The first encounter with the centaurs in Tomb Raider is considered a boss fight, due to the fact that they're the first Atlanteans you see in the game and their appearance is a shock. More centaurs are encountered later as regular enemies.
Done a lot in old beat'em-up arcade games. Examples: Double Dragon, Cadillacs and Dinosaurs, Ninja Combat, and Arabian Nights. Often, the first boss you face will also be the first Giant Mook.
As does the Streets Of Rage series with some of their boss characters. Lampshaded in Streets of Rage 2, with the weaker mook version of the boss "R. Bear" being called "Bear Jr."
2 takes it pretty quickly, too. After beating the twin robot bosses in Stage 7, the very first enemies you fight in Stage 8 are two more.
Averted by the original Final Fight and its SNES sequels: the bosses are unique to each stage and none of them actually reappear once they're defeated.
In the arcade version of Double Dragon, the elite mooks that appear near the end of Mission 3 and during the final battle in Mission 4 are all palette-swaps of Jeff, the Mission 2 boss (who was in turn, a head-swapped Lee brother). However, they're just as tough as the boss version of Jeff.
It also happens to Chin Taimei (Jeff's equivalent as the Mission 2 boss) in the NES version. In fact, it happens rather immediately, as the second fight in Mission 3 is against a group of three Chin clones.
In Super Double Dragon, Steve and Jackson, the first two bosses, appear throughout the rest of the game as mooks. All of the previous bosses also appear in the final stage.
In Knights Of The Round, Scorn (the first boss) reappears many times as an Elite Mook called "Tall Man." He's equally annoying, except he has less health.
Golden Axe and its sequels. Particularly noteworthy is Golden Axe II where after fighting four purple Hell Lizards as a boss, they begin returning on a regular basis (sometimes in the stronger green form). Of similar note are the huge minotaurs who appear as palette-swapped bosses twice then return as normal enemies! And remember, this is Golden Axe - there's no leveling up in this game!
Splatterhouse 3 has a big yellow ogre(?) for the first level's boss. The second level features two of them as normal enemies, and the third level has 2 green ogres. Dammit.
Splatterhouse 2010 has the Teratoid, a big monster with a tentacle for one arm that hits pretty hard and can regenerate other monsters. You later start encountering Teratoids regularly throughout the game.
The Streets of Rage series will often bring bosses back as mini-bosses in later levels. * Inverted in Dynamite Dux, where a miniboss that appeared in the first stage later becomes the main boss of the fourth stage.
Typically this will happen in fighting games if the endgame of the previous title in the series caused the main boss to lose their standing and are forced to fight against whoever took over to get it back. For the sequel, they will then become a playable cast member as they are considered a "participant" rather than the "organizer". Examples include Tekken's Heihachi Mishima between the first and second games (then again between the fourth and fifth games), Cervantes de Leon after Soul Blade, and Gaia after the first Battle Arena Toshinden.
In the second Marathon game, a Mother ofAll Hunters is fought as a boss on "If I had a rocket launcher...", then the final stage has at least three of them. On the Infinity level "You think you're big time?" they get re-promoted to boss status, and have homing projectiles this time. In 2, the King Mook version of the Cyborgs only appears once, while in Infinity it is a recurring enemy, although still rare.
Duke Nukem 3 D has Mini Battlelords, which are, obviously, smaller and weaker versions of the Battlelord. They're still a huge pain in the ass, however, since "weaker" only refers to their health (a mere 1000 to the big one's 4500) and not their tremendous damage output — worst case scenario, they can shred you in under three seconds. Gets even worse when they decide to spam their grenade launchers. And then there's the fact that in one of the later chapters there's one level where you end up facing three at once.
The climax of Half-Life 2: Episode Two pits you against a small army of biomechanical tripods known as "Striders", several of which made your life a living hell in a certain battle in the original Half-Life 2, and one of which was the actual boss of Half-Life 2: Episode One. Oh, and this time they have Hunter support. This battle would be quite a bit harder if you didn't have a car and a weapon that could One-Hit Kill them. Their machine guns receive a significant downgrade from the near-instakill they were in earlier installments.
For that matter, the Hunters were introduced with one nearly killing Alyx, and two or three of them was a boss battle early in the Episode. Two or three of them escort every Strider. Fortunately, you have a weapon that can One-Hit Killthem, too.
There's also the Antlion Guardian—when you first meet it (when you're instructed not to kill it, not that you can anyway), it has a poison attack. When you later actually get to kill it, this is missing, making it only a Palette Swap of the Antlion Guard. Not that it needs any help killing you all the same.
The Cyberdemon and Spider Mastermind boss monsters of the first Doom game return in the sequels as regular level monsters (though they're just as tough as they were as bosses), while the final bosses are upgraded to multiple-stories-high monster-spawning buildings. Doom 3 then promoted the Cyberdemon back to boss status.
And in the original Doom, the bosses of the shareware version (the Barons of Hell) become more common in the retail installments. Again, they're just as tough as they were as bosses, though the energy weapons (the plasma gun and the BFG) that you acquire later on make quick work of them.
In Doom 3, in an homage to the original Doom, you fight a pair of Hell Knights right before you teleport into Hell, and they too later become regular enemies, just as tough as before.
Doom 2 also adds the Hell Knights, a Palette Swap of the Baron with half the health. Combined with the above-mentioned double-barreled shotgun, they show up more than the Barons without unbalancing things too drastically.
Vagary, the first boss of Doom 3, later reappears as a mook, albeit a tough one.
The end boss of Heretic episode 1 is a small group of Iron Liches, who appear sporadically throughout the other episodes. The Maulotaur makes its first appearance as the boss of episode 2 and appears mid-level a few times in the episodes that follow. Interestingly, both of these creature types return as final bosses in the expansion - it has two episodes, and the first ends with a battle with a horde of Iron Liches, the second with one of Maulotaurs. Another way used of twisting the concept around is when a former boss appears at the beginning of an episode as a Giant Mook - and is more difficult than as a boss, because you just don't have the weapons and ammunition to beat it yet at that point.
Serious Sam: The First Encounter has the Aludran Reptiloid, Highlander, which appears as a boss. When it reappears in subsequent installments of Serious Sam, it has been demoted to a mook which is ironic since in the Second Encounter, they can withstand more beating before dying.
In Serious Sam 3, the first Adult Arachnoid is counted as a boss. It subsequently appears as a normal enemy and soon a smaller version of them also appears. The same goes for the first Major Biomechanoid, Technopolip, Khnum, and Witch-Bride you encounter.
Inverted in the "Legend of the Beast" DLC for The Second Encounter HD, which adds a stronger version of BFE's Khnum as the final boss.
The first boss in Will Rock is a huge Cyclops. From the following level onward you have to fight several other cyclops (their stone-spitting attack is different though). Then, the second boss, Hepheastus is later found as a King Mook on Mount Olympus. Several of them, actually.
Unreal does this with Titans; the first encounter is a locked-in boss fight (including an entire level essentially built around forshadowing it), then later on you just encounter them standing around in normal situations, as they are as strong as the first time and tend to come in multiples running away seems to be promoted. Your final encounter with one is another built-up boss fight with a much tougher version (which can be dropped into lava if desired.)
Plated Beetles, adult Sheegoths and Chozo Ghosts in Metroid Prime all first appear as mini-bosses, but later become regular enemies. The Sheegoth particularly feels degraded, as it was a quite hard to beat on the first time, but after you get the Plasma Beam, you can kill it with one shot without even waiting for it to expose its weak spot!
In Metroid Zero Mission, there is a Dessgeega that you encounter early in the game as a mini-boss after acquiring the long beam and discovering the currently locked entrance to the Big Bad's lair. This miniboss turns out to be a common enemy in a later level.
In Metroid: Other M, you first run into the FG-1000 security drones early on in the game, where they function as a fairly tricky miniboss battle. Much later, you find a few more, but by that time, you can blast through them with a single charged Plasma Beam blast, without even having to wait for them to expose their weak point, much like the Sheegoth example above.
In Halo 2, Tartarus, the Brute Chieftain, is the game's final boss, with a one-hit-kill gravity hammer and an invincible forcefield that can only be brought down by Sgt. Johnson's particle rifle. In Halo 3, Brute Chieftains regularly appear as King Mookminibosses; they're armed with one-hit-kill gravity hammers and one-use invincibility shields that last for a couple dozen seconds.
In BioShock, most of the game's bosses are simply regular enemy types with more health and the occasional attribute tweak (the Iceman in Fort Frolic is immune to ice attacks, for example). Most bosses are encountered before you fight regular Splicers of that type. For example, the game's first boss Dr. Steinman is a machinegun-wielding Leadhead Splicer, the regular versions of which you don't encounter until 3/4ths of the way through the game.
After you fight Jen in Prey, you will face it again as summoned mook when you're fighting the Keeper. And after you fight the Keeper, you will then need to fight agains more Keepers to get out. Fortunately for latter case, the Keepers will not summon mooks and there are leech gun ammo around.
The first boss of Descent 2, aka the "Red Fatty", later returns in Palette SwapGiant Mook form, armed with Mercury Missiles and Phoenix Cannons. In the first game, Fusion Hulks are a palette swap of the first boss that have Fusion Cannons.
Just about every boss in Painkiller was a unique one-shot, but the fan-made expansion Overdose abuses this to hell and back by taking the end-of-level miniboss from the first level of Painkiller and reusing him a grand total of over 50 times, with more than a dozen showing up for each individual encounter.
The Altered from Wolfenstein. The first time you fight one, you're armed with nothing more than small arms and have to use the environment to kill it. Later in the game you acquire a BFG that can kill one in a single hit.
Amsterdoom has this in every stage, with the boss of the area becoming a recurring type of enemy in the next.
In Quake, a pair of Vores are fought as a boss battle at the end of Episode 2, then they become recurring enemies in the last two episodes.
In the second game, the Super Tank and Tank Flyer first appear as King Mook bosses, then as normal enemies, although they're just as tough as before.
Inverted with the Uber Soldat in Return to Castle Wolfenstein, as you first fight a prototype Mook version, then a tougher boss version. Then played straight when the Uber version itself becomes a normal enemy alongside the Proto Soldats.
In Halo Combat Evolved, Zealot Elites were Boss in Mook Clothing-type enemies that were much tougher and more agile than normal Elites. In the second game, their shields were downgraded to the level of Major Elites and they only wielded Energy Swords, while their former stats (and then some) were given to the new Ultra Elites. Reach promoted the Zealots back to their former status and downgraded the Ultras somewhat.
Blood's bosses, excluding the final one, all appear as regular enemies in the episode(s) following their boss fight, though only above normal difficulty. Each one also has a subordinate version of themselves that appears primarily in the episode they're the boss of.
The Dark Forces Saga is rather fond of this trope. The Dark Troopers of the original game are introduced in this way, as are AT-STs in later installments. In Jedi Academy, in which lightsaber-wielding opponents are so numerous that they are basically Elite Mooks by the end of the game, an early level features one as a de facto boss.
The Butcher is an Overlord demon and a challenging boss early on, but later you can dispatch countless Overlords who are even stronger, albeit without the cleaver.
Zhar the Mad appears halfway through the game as a boss. Dark mage type enemies resembling him are later found in the final Hell levels.
Gorgons in God Of War. Medusa serves as the player's introduction to the enemy type as well as providing a demonstration of how to perform a special kill by ripping off the head. Every Gorgon you meet from that point onward (even the ones you face a few minutes later) is much, much stronger than she is.
The final stage in Zone Of The Enders: the 2nd Runner throws endless copies of Nephtis, the game's second boss at you, which you can cut down by the dozen with near impunity. Playing through the New Game Plus with the Eleventh Hour Superpower you get at the beginning of that stage will confirm that they haven't been made weaker than the original.
Used, in a fashion, in Mech Warrior 3 and the first two games Mech Warrior 4 series. Due to the presence of a stronger narrative structure in those games, various named enemy characters are present, many of whom serve as 'bosses' of a sort. As there is no character level structure in the series, the only thing determining the difficulty most enemies is what they ride, especially in comparison to the player's Humongous Mecha. Early on, players will often be in light or medium 'Mechs, and enemy heavies and assaults can be cast as 'boss' encounters. As the game advances though, what would have been a boss encounter is degraded to a common enemy, usually by virtue of the player having salvaged materiel that puts them on par with the enemy.
In Mech Warrior 3 for instance, the player starts in a basic medium 'Mech, and is tasked to fight mostly smaller, less well armed or armored machines. The first 'boss' fight is a Mirror Match against an enemy using the same model of 'Mech as the player, and later boss characters often involve the introduction of dangerous heavy or assault 'Mechs at the end of a mission. By the later missions, though, the player's starting Mech is plainly outclassed, but the enemy will still use that chassis, and they become little more than speed bumps by that point.
Mech Warrior 4 also features this. The player starts in another medium Mech, but will fight against superior heavies and assault class machines as named encounters, including going up against several Lightning Bruiser designs. Come the end of the game, though, even those designs are not the challenge they once were.
In World Of Warcraft, Kael'Thas Sunstrider starts off the first expansion as one of the final endgame bosses who required a group of 40 players in max level gear to reliably take down. Two major updates and one necromantic encounter later he's still a final boss... but of a dungeon that any 5-man group of newly-max-level players can take down without too much trouble.
The MMORPG City of Heroes and its Villain counterpart do this as well in a few cases. Some specific missions have Boss or Elite Boss versions of enemies that are normally only minion or lieutenants normally, while in other cases Bosses are eventually downgraded to lower status. Villains can run into this as early as level 5 and the Lt. Blechley enemy, an Elite Boss version of the normally lieutenant-powered Council Vampyri. Heroes encounter this as well, although the most notable case doesn't come into play until level 45, where the dangerous Malta Gunslingers begin to regularly apply as lieutenants instead of their previous Boss counterparts.Justified as heroes don't even start to encounter the Vampyri until level 20, while villains encounter Lt. Blechley before level 10. At that stage in the game, he would be tough for a hero as well.
Guild Wars Nightfall, a powerful boss early in the game is a construct made of floating stone fragments called the Apocrypha. In the final third of the game, identical creatures appear in large group as regular encounters, and are called "graven monoliths."
And in Factions, the first mission on the mainland has you fighting a Shiro'ken boss at the end, while by the second to last mission, the Shiro'ken are essentially elite mooks.
Mabinogi has an interesting example: "Boss" status refers to a monster being so many levels above you, and while dungeons have unique boss encounters, you'll come across many "Boss" monsters outside which will eventually downgrade to normal and "weak" after you've leveled sufficiently.
Grand Chase has quite a few of these. The first is the Orc Warrior, who is degraded to a normal mook a mere two stages after his appearance as a boss. Then there's the Troll [Two stages again], Lich, Gorgos, and Paradom, although these appear as Mini-Bosses. The Hero Dungeon also has buffed versions of Gaikoz, Gardosen, Kamiki, Giant Stone Golem, and Basilisk as minibosses. Multiple at once.
The Butcher in Rusty Hearts starts out as the boss of the Subterranean Canals B2, but turns into a regular (though slightly more powerful than normal) enemy on the Hard and Very Hard difficulties in Wine Cellar 1F.
In fact, all Butcher-type enemies (Armored Butcher, Hammer Butcher, Gloves, etc.) become degraded bosses later on in the game. In Blood Mode versions of certain levels, you'll fight at least one Butcher in every room before the boss.
The boss of the first level in The Lion King video game is a hyena. They are demoted to "mere" borderline-Demonic Spiders in the Elephant Graveyard and even further to common Mooks in the later adult Simba levels.
Castlevania games have been doing this since the beginning, when the first level boss, the Giant Bat, reappeared multiple times on the last level.
The Giant Bat from the first game is echoed by the Armor Battler in Castlevania: Bloodlines; several instances of it appear in quick succession in the last level, with heavily reduced hit points.
Castlevania The Adventure has the first boss appear a few times on the final level as a normal enemy. It has less health than when it was fought as a boss.
Several early bosses in Castlevania: Symphony of the Night (Gaibon and Slogra, Karusuman, the Lesser Demon, plus the arena's Werewolf and Minotaur). The degradation seems to have stuck, as several of these monsters have remained normal enemies in later games in the series.
Aria of Sorrow loved this. The first four bosses become regular enemies later. There's even more powerful versions of these enemies later on.
The Man Eater is an inversion. It's a regular enemy that shows up in the Boss Rush mode for some reason. Order of Ecclesia brings it back as a full-fledged boss.
Also in Aria Of Sorrow, there is a combination of this and Bait-and-Switch Boss. You enter a boss room and the Giant Bat appears...And is promptly crushed by a giant hand. You still get the Giant Bat soul after beating the real boss.
The Frankenstein's Monster Boss has bounced back and forth from boss, to normal enemy, to boss over many of the later games.
Order of Ecclesia had it both ways, with several palette swapped Frankenstein's Monster types as normal enemies (The Creature-which is the name the original boss and most normal enemy incarnations go by, Enkidu, Rebuild) and one as a boss (Goliath)
The Giant Skeleton in Order of Ecclesia gets this treatment, in the "two-at-once" variety. Considering that this Castlevania is a good deal harder than other recent ones, and everything in the game had a good chance of killing you anyway, this isn't surprising.
The Stone Golem and Werewolf have also bounced between Mook and Boss status from game to game.
Portrait of Ruin also uses a lot of degraded bosses. Many of the bosses from Rondo of Blood (Including Minotaur, Wyvern, Dogether, and Camilla's assistant Laura the catgirl) appear as normal enemies, as well as the aforementioned Slogra and Gaibon. However, the Werewolf and The Creature are back to being bosses again - but 2 of The Creature appear in the Nest Of Evil. And you better believe they're not degraded whatsoever despite not being the floor boss(es).
The Dullahan enemy moves up from Mook to Wake Up Call Boss in Portrait of Ruin, only to go back to Mook just one game later in Order of Ecclesia. Though the mook version is often called Durhan.
Series wise, the boss version could be a King Mook variation. Before and after, it's a mook, that one time it's a boss. However, they appear as mooks again in Harmony Of Despair, exactly the same as in Portrait of Ruin (giant, hard to get around with projectiles that curse, and you have to hit a floating head).
The Giant Armor in Harmony of Dissonance appears as an enemy later on.
Many of the early bosses from Lords of Shadow show up as pallete swapped recurring mooks by Chapter 2. The first boss shows up as a Boss in Mook Clothing as early as the second level.
In the sixth stage of Ghosts N Goblins, the bosses from stages 1-4 show up again as regular enemies that can be bypassed without fighting. They're no weaker than they were earlier, though.
In Mega Man X 8, various enemy reploids in the final stage use copy chips to turn into duplicates of Sigma, the heroes' recurring nemesis (and final boss of the first seven games). Fortunately, they only have access to Sigma's original body, not any of the nightmarish battle forms he loves to inflict on the player.
In Battle Network and Star Force, after defeating a boss, they'll have a ghost version hiding on a specific tile somewhere in the Cyberworld / on the Wave Roads, usually in a dead end or a corner of a wide platform. Step there and a stronger version of the boss will appear to fight you. After you defeat it, an even stronger version becomes a rare random encounter, usually in the same area.
In Mega Man 9, the second Wily Stage's boss is a ship that has three parts, each with their own health bar and attack pattern. The battle is pretty tough, especially because it's a Wily boss, so you have to conserve weapon energy. So what do they do? They take the already super-toughdownloadable Fake Man stage and stick in the ship along with several mini-bosses. Not to mention that because it's a Time-Attack-only stage, you have only one life, and no E-tanks. What you basically have is a Degraded Boss who takes this trope in a bunch of different directions. Better hope the enemies drop some energy pellets, because you'll need them.
The recent gameplay videos for Rockman Online have shown old bosses such as Stone Man as normal enemies.
After fighting Botos in Mega Man X Command Mission, he sends decoys after you a bit later in the game, which are notably weaker than Botos himself. When you actually fight the real one, he just beats you up for a little bit and then runs away.
In Sonic Adventure 2, some of the stages operated by Eggman contained enemies that were near-duplicates of E-102 Gamma from the previous game. Interestingly, in Sonic Adventure, Gamma was both a player character and a boss, and made occasional appearances as an NPC, so he ends up having had quite a varied career.
There's also the fact that in Sonic Adventure 2, Bigfoot troops showed up as the first bosses for the hero and dark side story archs. When they show up again in Shadow the Hedgehog, they are reduced to standard enemies in the stage 5 and stage 6 options.
Keith Courage In Alpha Zones brings many of the earlier bosses back several times, as palette swaps, two-at-once bosses, and infinitely recurring enemies.
Fire Leo, That One Boss for many players of Viewtiful Joe, reappears in the next chapter as a normal enemy called Metal Leo with severely reduced health and no fire attacks, thankfully.
The regular assassins in Iji appear after you fight Asha. Their tactics are less showy and give more time between attacks but also gain an attack that Asha couldn't perform because he only has one arm. In fact, nearly every boss enemy or notable character is just a normal enemy with some backstory and an optional supermove (and more health).
Midnight Wanderers, one of the three games forming Three Wonders', has the flamethrower from the Terror Twins (Stage 2 Boss) and Dumpty (Stage 3 Mini-Boss) reappear later as minor mooks.
In Spyro this happens on occasion. The best example would be Buzz, the first boss in Spyro: Year of the Dragon. When he first appears you have to knock him in lava six times to kill him, and each time he gets out you have to dodge his roll attack (and the last two times he gets out he has a powerful fire breath). Cue the third boss, Scorch, who spits Buzz out as an egg. After being hatched, Buzz still has to be knocked into lava/acid, but only once to kill him this time, and you don't have to deal with his roll attack or fire breath.
In The Caverns Of Hammerfest, every single 'boss', with an exception of the Final Boss, are rather introductions to more powerful enemies you will find in the next levels.
The Daemon mini-bosses in the SNES and Genesis game Warlock appear as regular enemies two levels after they're introduced. The player hasn't gained anymore power, and they have the same amount of health, these are still pretty much Demonic Spiders. The only difference is that they are now skippable. The fact that this stage is That One Level doesn't help matters.
In Pikmin 2, the Burrowing Snagret — the boss of the third dungeon you visit — appears as a regular enemy later on in the game (and not that much later), and often in pairs. The Emperor Bulblax, which was the Final Boss of the previous game, appears also as a boss in one dungeon, but later on occur as mere mini-bosses and in pairs.
The Binding of Isaac takes this Up to Eleven. By the time you get to Sheol, all of the pre-Wrath of the Lamb non-final bosses except for Gurdy can show up as mini-bosses in any room and the Seven Deadly Sins can show up in any room. In The Cathedral, Wrath of the Lamb bosses are degraded, as well. The Chest, The Very Definitely Final Dungeon, contains rooms with either chests or Degraded Bosses, and bosses that were never degraded before, like the aforementioned Gurdy, are now degraded. barring the True Final Boss. And these are the only possible rooms.
Final Fantasy VI has an interesting example. Most of the monsters seen on the Veldt are just regular enemies. However, you can find both forms of Sr. Behemoth and the Holy Dragon (one of eight allegedly unique dragons) after killing them normally and get their rages and the items they drop upon defeat.
Not to mention the M-Tek Armors. You fight two of them as a boss when you first get Edgar (though two shots from Edgar's crossbow will kill them.) Then, in Sabin's scenario, they're all over the place in the Imperial outpost as forced and avoidable non-random encounters and easily fall victim to Sabin's Blitzes or Shadow's shurikens, especially after you get Magitek Armor Suits of your own (and you can use a Bolt Beam to attack their weak points for massive damage.) You can also find them on the Veldt, though Gau starts with their (useless) rage.
Every single boss in Final Fantasy: Mystic Quest, with five exceptions: the crystal guardians (and their Palette Swap counterparts in Doom Castle), and the final boss.
Final Fantasy VIII: Granaldo and its Raldo minions are fought on Disc 1 as a regular boss, but they appear in Disc 2 as pets of the Garden Faculty.
A few bosses, namely Granaldo and the Oilboyles, appear in the final dungeon as regular enemies.
Final Fantasy IX: The Four Chaoses (Lich, Marilith, Kraken, and Tiamat) show up as regular bosses in specific points throughout Memoria. However, weaker, "Crystal" versions of them appear later in the Crystal World as random encounters.
The Bonus Dungeon in Final Fantasy X 2 has a lot of these. In the 100-floor dungeon, you encounter bosses on every 20th floor, and after beating the 80th floor boss, all the level bosses you've beaten thus far will start showing up as normal enemies beyond the 81st floor. Considering how incredibly powerful they are (the last two have over 10 times the HP of the final boss!), it's generally a VERY good idea to run if you run into them, as they're NOT worth the effort to beat a second time.
Some bosses from earlier in the game also show up later. If you revisit the Floating Ruins in later chapters, you will occasionally encounter Boris, the boss from the first mission there. He has exactly the same stats as last time, therefore making him much easier to defeat.
Ultima and Omega Weapon, usually Bonus Bosses, have been relegated to random encounters for this game. Surprisingly, Omega is the easier of the two.
X-2 also has bosses from Final Fantasy X return as regular encounters. Because the mechanics of the games are so different, they can be much easier or more difficult then your remember.
Final Fantasy XII plays it straight with the Urutan Eater—reappearing as the Emeralditan in the Nabreus Deadlands and Garuda, which reappears as the Garuda-Egi in Paramina Rift, and inverts it with the Rogue Tomato, a level 3ish boss who reappears as a much bigger pain in the ass called the Deadly Nightshade, and the squad of five Mandragoras, initially an easy and fun boss fight and then later reappearing (if you can spawn them, that is) as regular enemies that are very good at making the gamer hate his or her life.
Final Fantasy XIII has several enemies that count in this category, such as various Behemoths and Wyverns. Of particular note is the Juggurnaut, a giant mechanical monstrosity that can and will tear your team to shreds should you stumble into it early in the Pulsian underground. However, when they appear later, your party will be strong enough to turn them into scrap metal in about a minute.
Final Fantasy XIII-2 has a couple cross-game inversions: The Immortal and Ochus were regular, if strong and rare, enemies in the first game, but are unique Bonus Bosses here.
Vagrant Story featured bosses that often become normal mooks with varied stat decreases. The Harpy and Lich both appear as mooks a few rooms following their boss arenas, though, unlike other examples in the same game, has their stats decreased.
Many, many bosses in Lufia And The Fortress Of Doom. With the exception of the Sinistrals, you don't fight an actual unique boss until around the halfway point of the game.
Lufia II Rise Of The Sinistrals loves this as well. The general rule of thumb there is, "if the boss doesn't have any lines of dialogue and the game hasn't hit its 3/4th complete point, expect to see it in normal dungeons regularly later on."
A few minor bosses such as the Desert Axebeaks in Legend Of Mana are simply normal enemies with a boss-level hit point bar, although due to the game's level scaling system these downgraded enemies can eventually become more dangerous than their Boss equivalents.
Shin Megami Tensei II features an example of this that's notable for the rare case of having obvious justification. The enemy Betelgeuse first appears as a boss... and a giant monster. Later, you encounter him as a normal enemy, but in this form, he's human-sized and humanoid.
Any Mega Ten game with the recruitable demon mechanic will allow you to recruit, fuse, or otherwise obtain many of the bosses you fight after beating them at least once.
In EarthBound, the Kraken, which was fought on the way to Scaraba, reappears in the Sea of Eden (where Ness has to fight them alone). It then reappears again in the Cave of the Past as "Bionic Kraken". Also, several other enemies in the Cave of the Past are renamed and palette-swapped versions of previous bosses (such as of Starman DX and Boogey Tent).
In Dragon Quest, you must rescue the princess from the Dragon early on. Later, dragons are all over the place. The Axe Knight also first appears as a boss guarding Erdrick's Armor, then as a recurring enemy in the Final Dungeon.
In The World Ends With You, there are various lower-level Noise (examples including the lowest form of wolf, shark, and rhino), which when you encounter them for the first time, they're treated as a boss. Later in the game however, they're just another type of Noise to defeat - sorry, erase.
Super Mario RPG presents a spin on this: several of the game's earlier bosses reappear in the game's final dungeon, a factory that literally churns them out. They're even called "Machine Made," no matter what boss they're copies of. Every last one of them is called "Machine Made."
Most Machine Mades are somewhat to significantly easier than their boss counterparts. The exceptions are Machine Made Mack and Machine Made Bowyer, who both have more HP than their "original" forms.
The Machine Made Yaridovich has a unique evasion trick where it splits into four lesser mooks and you need to pound away at them until the original Machine Made returns to the field. Like the original's Mirage attack, it gives your party more enemies to be assaulted by; unlike the original's Mirage attack, these enemies are easily dispatched by your party at its current strength.
Unlike most examples of this trope, the boss music still plays while fighting Machine Mades, interestingly.
Persona 3's "The Answer" more then likely caused some episodes of PTSD when one of the most challenging bosses from the main game, The World Balance, showed up as a regular encounter with all its moves and stats intact.
Persona 4 has a (sub-)boss with a really quick turnover, the Avenger Knight (who can kill an unprepared party real easily) appears on the floor right after his initial appearance, sometimes appearing in pairs! Thankfully these have a fraction of the HP and none of the physical skills the boss had and can be one hit killed 100% of the time via Hama/light, easily found on Angel at this point
A lot of bosses from P3 can be found as random encounters in P4. World Balance shows up again, but it's an interesting case; you find it as a random encounter way before you fight it as a sub-boss. Fighting it then gives you a version that resists physical attacks as well as all elements, but falls instantly to Hama and Mudo and only knows Ziodyne. Later on, you fight a powered-up version in Nanako's dungeon, which still isn't as hard as the nightmare it was in the previous game. Here, the World Balance has a simple pattern (Mind Charge and then a -dyne spell) and has low HP for a boss.
The Magical Magus appears in the first level of the game, and in P3, you didn't fight it until you got to the third block of Tartarus.
The Natural Dancer also makes an appearance. It only has two skills now; Navas Nebula (Physical attack, whole party, causes Exhaustion, which drains SP each turn) and Marakunda (Debuff, lowers defense of all enemies). It also picked up a weakness to ice, which it didn't have in the previous game.
In Fable II, even The Dragon is not immune to this; after you kill the Commandant very similar looking people who have the same powers and abilities appear. This is at least a justified case, as the Commandant is introduced as a prototype, and similar creations are explicitly being mass-produced during the 10 year interval at the game's halfway point.
Also the boss Thag the Impatient later returns as common Bandit chiefs.
In Fable III, downgraded versions of Captain Saker and Lieutenant Simmons (deceased) appear once you've defeated them and are a high enough level.
The Giant Snake, a difficult Action Commands boss from the first Dark Cloud, appears as a regular (albeit hard) foe in Dark Chronicle's second area. The level where it first appears in is even named after it.
The first Kingdom Hearts does this with the Behemoth Heartless; after appearing as the 'boss' of your second visit to Hollow Bastion, you fight more of them in the Hades Cup and the End of the World, only this time they're basically Giant Mooks. Stealth Sneak also appears as this in the Hades Cup but subverts this by being stronger than the original, and you face two of them at once. This trope becomes quite egregious in 358/2 Days, with most of the targets for missions becoming lesser enemies later on; one of the later missions involves defeating six mini-bosses.
The first game also has the very first boss, a gigantic knight split into several pieces, appear again in the arena. Except only parts of it appear, so it's very odd to see a random armored leg or arm fighting in the arena.
The Tyrannosaurus Rex looking Dinosaur from Dragon Ball Z: The Legacy of Goku that were immune to Kamehameha. Originally beating the first one took half of this player's Gameboy battery power, it took so long. After the first one, more Trex look alikes show up in later levels of the game, each time easier to kill as Goku gets stronger until he's too fast and strong for the dinos to even be worth the time.
Adventure Quest does this a lot with war bosses, which in many cases end up in random encounter lists. This gets lampshaded in one quest where you have to fight Drakath the Undead Dragon, who it's likely the player has run up against at least 3 or 4 times before, and their character wonders, "Who keeps reanimating that dragon?"
Wizardry: Tale of the Forsaken Land contains several examples.
On level 4 you fight an enemy and minions as a mini boss for an optional quest. By level 6 these are normal enemies.
On level 8 you will fight a mini boss in a certain room that due to timing and various other factors makes this the hardest fight in the game to get right at the time you get there. And it's just an optional quest mini boss with near zero plot importance. On the final level of the game you fight this exact enemy as a normal encounter with the same minions... and may even encounter it at the same time as another difficult enemy. Post game you will sometimes fight two at once and minions as a normal encounter.
For that matter, post game you can fight bosses as normal enemies. Even the ones that actually were bosses.
In Digital Devil Saga, Jatayu and Garuda are fought on the outside of the higher levels of the Karma Temple. After being defeated and going back inside the temple they can be encountered in random battles.
Wendigo in Devil Survivor. It's still referred to as a fearsome demon in cut-scenes even when you're strong enough to take on five teams of Wendigos at once. It's actually even invoked, as one of your party members will mention how Wendigo appearing as a regular enemy means that you're fighting tougher demons now.
The Barbarian Fighter and the Iron Golem from Summoner. The first can be fought in a normal encounter practically right after you fight him as a boss, and the Iron Golem can be found as enemy before he's a boss, if you're a glutton for punishment and go wandering in the mountains.
The first boss you face in the main storyline of Dragon Age: Origins is an ogre, which becomes a normal enemy later in the game (with more powerful versions at times.) Justified in that you're trying to retake an area that's off to one side of the main battle, and the darkspawn would've just sent a strike force consisting of their normal troops to take it in the first place.
The Dynamic Difficulty of the game scales all monsters to the Player Character's level, with Normal enemies being slighly weaker than the PC; Lieutenants, slighly stronger; and Bosses, about as strong as your entire party combined (don't ask about Elite Bosses). Said first Ogre you encounter (at level 4-5) is a Boss, yet all subsequent Ogres are Lieutenants, so they are simultaneously stronger that the first one and weaker than yourself during all subsequent encounters.
Same thing happens in Dragon Age II: an ogre is the very first boss you fight in the game (twice), and even Flemeth comments on how impressive it is to beat one. The next ones you meet in the end of Act I are considerably less difficult. By Act III, Hawke and Companions will crunch them for breakfast.
Heck, Origins did that with freaking High Dragons. The Mountaintop Dragon is a boss so obviously overpowered that the game almost asks you "Are you really, really sure?" several times before you are allowed to fight it (and awards you an Achievement if you prevail). The Expansion Pack puts a High Dragon to guard the entrance to The Very Definitely Final Dungeon—whom you rip apart before you even register its creature type. Twenty extra levels do make a difference.
In Origins and Awakening, Pride Demons are one of the few bosses dangerous enough to present a challenge even when not backed up by waves of flunkies. In DAII, however, they get Nerfed so hard that a two-character party can take one out without breaking sweat unless it's backed up by wave upon wave of Mooks.
Crisis Core: Final Fantasy VII is full of these. Sometimes they're storyline bosses, sometimes they're mission-ending uniques.
In Skies Of Arcadia, the executioner you face fairly early in the game appears reskinned as a mook in one of the climactic sequences. The original inspired nightmares, his green-painted cousins are easy meat, probably because you gained about 30 levels and now have access to Fina's wide range of healing powers.
In Xenogears, you first face off against Redrum, who is one of the game's top contenders for That One Boss. A bit later, you fight Bloody, a palette swapped version that has higher stats, but is weaker relative to your higher level party. Then, late in the game, there are random encounters with a pair of Bloody Brothers. They're identical to Bloody, except they use Redrum/Bloody's trademark attack, "Murder" much less often, and by this point in the game all their other attacks only deal 1 damage.
Xord in Xenoblade Chronicles is the first Faced Mechon you fight proper. Later on Mass-Produced Faces that look and fight exactly like him show up as regular enemies. One of the few examples that are entirely justified in-story though, as there are implications that Xord is a Mass-Produced Face himself, meaning the minor antagonist you assumed was The Brute turned out to be just a run-of-the-mill Elite Mook who happened to have a backstory.
The Lotus Assassins of Jade Empire get this hard. The first one you meet, you can't even fight - an NPC rushes in to defeat the assassin and says you wouldn't have stood a chance. Later, you fight a couple, but each is at the center of a boss fight. A couple of acts later, and you're taking on entire squads of assassins by yourself without breaking a sweat.
Ascended Sleepers in The Elder Scrolls III Morrowind. Various named Dagoths encountered in the latter half of the main quest are modified Ascended Sleepers, but they're actually downgraded from the normal enemy (which only shows up at extremely high levels - it is in fact the highest levelled non-unique monster in the game). So it is quite possible to learn to hate Ascended Sleepers before knowing what they're actually called.
Secret of Evermore had the Eye of Rimsala; the first is a boss about halfway through the game. The last dungeon area has one guarding most corridors, and the last Sequential Boss fight includes a segment where you take on three at once.
Legend Of Legaia has you fight against two Viguro early in the game. They're second-tier Seru while you're still on first-tier spells. Interesting in that it is still possible to absorb them in this fight if you're lucky enough, giving you a considerable boost in damage output for as long as you can meet the higher MP requirement.
The Kemaro mini-boss fight is a lesser example.
Priel from Luminous Arc gets smacked with this hard. In the first game, she's The Dragon, and a royal pain every time you fight her. In the sequel, her sprite is reused as a generic ranged Mook you'll see in the first five chapters.
Star Ocean Till The End Of Time does this with nearly every single boss in the second disk. They don't wait for the player to get stronger before showing up again, nor do they get weaker upon being degraded, they just get a little stronger and show up as normal enemies from then on out.
Star Ocean 1 has a case of a boss being degraded before you fight him. It's the Velcant. It's a rare random encounter in his dungeon, so some people don't run into it. What's worse is it's a Wake Up Call Boss, one that either forces you to grind or forces you to learn how to effectively use the battle system and control the party rather than just mashing A and let the AI control your other party members.
The Great Jaggi is your first large Monster Hunt in Monster Hunter Tri. The first time you encounter one wandering about Moga Woods (usually the very next day) is probably a bit of an Oh Crap moment. By the time you fight the Royal Ludroth, you're taking them on two at a time, and they're barely worth hunting anymore. Note that at this point, you're still fighting the weakest monsters in the game.
Breath Of Fire makes recurrent use of the trope in their early entries:
At least in the Super Famicom game, Dragon Ball Z: Legend of the Super Saiyan, there are random encounter enemies late in the game that are palette swaps of Cui, Dodoria, Zarbon, and the Ginyu Force that are weaker or stronger than you originally fought as bosses.
A few bosses in Shin Megami Tensei III Nocturne are fought in later dungeons as random encounters. This includes Forneus, the Troll (although he was only a miniboss to begin with), Ose, Yaksini (same as the Troll), and a few others.
In Dark Souls, the Taurus Demon, the Capra Demon, Pinwheel and the Moonlight Butterfly can all be encountered as normal, respawning enemies later in the game.
The Taurus Demon is particularly bad about this as with the others, you can sometimes fight them one on one, and in a relatively safe environment. The area where you first see the "Lesser Taurus Demons" (which actually have slightly more hp then the boss normally did) you see them in a recently cooled down lava lake, with still bubbling lava. oh, and theres you know, seven of them.
The Bell Gargoyles also reappear as non-respawning mini bosses in Anor Londo, although they're smaller and breathe lightning instead of fire. They're also encountered individually, which makes them much easier to deal with.
The Sanctuary Guardian boss in the Artorias of the Abyss DLC becomes a degraded boss after you rescue Dusk of Oolacile. Two of them appear in the arena where you fought the first one, but they have much less health.
In Mass Effect 2, an YMIR heavy mech is the first boss. As the game progresses, they show up frequently as elite mooks, sometimes in pairs. They're never easy, and in the harder battles (especially when they start out near the party, since they can absolutely shred Shepard at short range), they qualify as Boss in Mook Clothing, and are sometimes harder than actual bosses.
InMass Effect 2, the boss on Therum is a Krogan Battlemaster. You fight them later in the game, but they are much easier.
The first Brute in Mass Effect 3 counts as a miniboss, complete with arena-like setting and cinematic introduction, but others will be introduced later in the same mission with much less drama. Ditto for Banshee later in the game.
Also in the third game, the Atlas Mech is counted as an outright boss when encountered during Priority: Sur'Kesh and Priority: Eden Prime. They become more common in later missions though.
Inverted with the Geth Colossus and Thresher Maw. In the first game, you'd probably end up killg a dozen of each due to the fact that you had the Mako IFV. However, in the second game, only one of each is fought; since you're fighting them on foot this time, they're treated as bosses.
Happens in Pokémon. You'll be fighting trainers with Com Mons, and the gym leaders have evolved Pokémon, or Pokémon not available to you yet. Eventually you'll be able to find evolved Pokémon, or stronger unevolved Pokémon both in the wild and used by trainers.
The first boss you fight in Shin Megami Tensei Strange Journey is Orias, who shows he's serious by killing off several Strike Team members and Commander Gore. He's level 7, and can't do anything but attack. A few sectors later, Orias is a random encounter... and he's level 27 and throws Magarulas around like they're free candy.
Almost every boss in Tales Of Phantasia reappears later in the game as a standard enemy, even such important bosses like summon spirits (Undine and Volt in particular).
Big Core in the Gradius series as a whole; they were the first game's Recurring Boss and came back for Boss Rushes in subsequent games. In Gradius V, they've been reduced to regular enemy status, appearing frequently in Stage 1, 3, and 7 and getting killed pretty quickly.
Another level boss Gaw appeared in Life Force at the end of Stage 5. It appeared in Gradius II's Boss Rush before becoming a pre-stage enemy for the bio levels in Gaiden and V.
The giant worm that was the first boss of Gaiden appears as an enemy in V's bio stage.
The worst is Zelos Force, the final boss of Salamander/Life Force, degraded to a simple obstacle course of multiple Zelos Forces in Stage 1 of Gradius V.
Darius Twin's final stage is this trope incarnate. There are no normal mooks to be had...every last thing you run into is either a mid-boss or stage boss you've encountered prior. And they will swarm the screen to no end.
Two variants occurs in Contra ReBirth. The giant alien worm faced as the first boss of the game becomes "ammunition" for the penultimate boss (or Final Boss if playing on Easy), Uranian Devil Gaba / Jagger Froid, serving a similar purpose to the Recurring Boss's appendages in the other games. On the same boss stage, another Recurring Boss that served as the Final Boss of Super C is reduced to mini versions on a stampede.
Several of the mooks in Fester's Quest are miniature versions of bosses from Blaster Master; both games were by Sunsoft.
Space Invaders Infinite Gene is insane with this, with a hundred or so of these in the normal game (of 30 levels) alone. The degraded versions usually have less health, come with different enemies\obstacles each time, and they'll go away if you can't beat them in time.
Yuyuko, final boss of Perfect Cherry Blossom, is the Stage 1 boss of Ten Desires. Justified in that she's Willfully Weak; she just wants to get in some practice with you before you go on the investigation.
The fangame Concealed the Conclusion is full of those: Flandre and Mokou (Bonus Bosses in original games) are stage 1 minibosses, Kaguya and Mima (originally Final Bosses) are stage 1 bosses, powerhouses like Remilia, Eirin, Shikieiki and Yuuka are on stages 2-3, etc.
Generally speaking, if a characters recurs they'll be showing up a midboss the second time. Though this can be a promotion if they went from a boss of an early stage to an EX midboss.
In Raiden IV, the twin spider tanks from Raiden II return in smaller form in Stage 4, but then they are re-promoted to bosses in that same stage. The series' recurring jet boss, Ichneumon, becomes a regular enemy in Stages 5 and 6 of the Xbox 360 version.
The survival horror game Silent Hill 2 miniboss, the Doorman (or Abstract Daddy) appears as a common monster in a later level, albeit a smaller, weaker version of the original. The reason for this being, based off the most popular theory, that the psychological reasons for the original creature to exist are sort of "echoing" themselves. Or something.
Similarly, in the third game, the Missionary, which you first fight as a boss after one of them kills Harry, becomes a recurring mook in the cult's church near the end of the game.
In Origins, Caliban, the boss of the theater, becomes a mook in the outdoor segment almost immediately after you beat the boss version. Along with the giant versions of the dogs, these act as a very, very unsubtle hint that you should probably just be running from enemies on the streets now instead of wasting ammunition and Breakable Weapons fighting them.
Los Gigantes from Resident Evil 4 undergo an interesting form of degrading. When you first encounter one, El Gigante is a ridiculously powerful boss that you only manage to beat because you have lots of room to maneuver and (hopefully) a dog to provide a distraction. The second time, you lack this room, and you're expected to use the terrain to delay it long enough to escape. By the third and final time, you've got enough firepower to handle two Gigantes with relative ease — and since you retain your arsenal when you start the game over, from the second round on Los Gigantes are pushovers from the start.
It kind of helps the 3rd time that you can activate a lava pit and eliminate one of them quickly. In true RE tradition, Convection Schmonvection applies.
The Prototype Tyrants in Resident Evil 0, although this is more of an inversion, as the game is a prequel.
The Tripod from Dead Space 2 may be a challenge when you first fight it but it has to attack in increasing numbers to be a threat latter on. One actually runs away from its own boss fight.
In Gears Of War, the minigun-wielding Grinders in the second game are functionally weaker versions of the first game's final boss, General RAAM, with less health and no Kryll Shield. Story-wise they're two entirely different beasts (RAAM being an ascended Theron Guard while Grinders are big dumb Boomers), but gameplay-wise they're very similar.
In the 2004 Transformers Armada game by Atari (Good luck finding it), the boss of Level One is a "Heavy Unit" wielding dual energy blasters, homing missiles, and a mean Shockwave Stomp. Oh, and you fight him in relatively close quarters (with some terrain for cover), too. He becomes a standard Giant Mook no later than than level two, when the game reveals its true Nintendo Hard colors: They can survive one or two Boom Headshots from your Sniper Rifle (depending on your aim), and are frequently stationed out in wide open areas where they are free to launch their homing missiles at you from really long range. And that's not counting that nearby reinforcements will wonder what they're shooting at and start searching for you themselves. Fun game though.
Hunters in Prototype, although they don't degrade by a lot until much later in the game. It isn't that they get weaker, it's that your character grows in power a lot. Fighting them is still very tricky, until you get the hunter dirtnap ability (which lets you chokeslam them). Once you get the blade power, however, they fall in one hit.
MDK 2. You beat and destroy a Minecrawler Pilot as the very first boss of this game. Level 7? You end up fighting at least five as "regular" enemies.
Max Payne 2 has Kaufman, the much-feared leader of the Squeaky Cleaning Company hitmen, whose baseball cap and jacket set him apart from his jumpsuit-wearing lackeys. In the levels following his death, Cleaners wearing his outfit are only a little less common than the standard models.
Dirge of Cerberus does this a lot, most notably with the Heavy Armor Soldier mini-boss in the third level.
Fire Emblem The Sacred Stones has the Cyclops, fought as a regular boss previously, appear as a regular enemy in the Endgame and Bonus Dungeons. It's as tough as before though; it's your party that's become stronger.
Proving it's never too late to pull this, the boss of the second-to-last chapter is a Dracozombie. You have to fight two as regular Mooks (though they're more Bosses In Mook Clothing) in the final chapter, and even more appear in the Bonus Dungeon. Interestinly, the first one you fight has a radically different sprite... because he's Morva, a very significant NPC and Myrrh's father, suffering a case of Came Back Wrong.
Happens a lot in Fire Emblem. The first bosses are different classes. As you get some of your own, the bosses become higher tier classes, with the mooks being like the first bosses. Get some of your own of that too, then by the end of the game all the mooks swarming each level could have been a boss of an earlier level.
Shining Force used this often, with several bosses on stages being a Minotaur, a Golem, a Witch creature, a Black Knight, etc., all of whom would appear frequently as simple mooks in later missions, once the team got stronger.
Shining in the Darkness had the Kaiserkrab, the insanely hard-to-beat first boss. When it reappears as a mook, it's just as powerful but easier to beat, since you have two extra teammates by then.
The special zombies in Dead Island show up initially as bosses, complete with cut scenes showing shocked protagonist reactions. After couple of areas have been passed, they show up mixed in with regular zombies, as strong as they were before (stronger, in fact, as they level with the players). As they tend to have elements of a Puzzle Boss about them (disable the arms first, use ranged weapons, only attack from behind etc) it's usually best to mop up the mook before attempting to take them down.
The third one doesn't even bother waiting until the second is down.
You know it's bad when Ganon gets this treatment.
However, it's inverted in the Metroid Blast attraction. The first time you face Ridley, he gets no fanfare and is treated as a normal enemy. Every time you face him after that, he gets an intro cutscene reserved for bosses.
After facing Ranba Ral (Gouf) and the Black Tri-Stars (Dom) in Mobile Suit Gundam, the Mobile Suits used by them are soon adopted as Mass-production units by Zeon Mooks.
Similarly, in Gundam SEED Destiny, The Earth Alliance's powerful Mobile Armors such as the Zamzah-Zah and Destroy Gundam appear in greater numbers later in the series (and are usually killed much easier than the first one they faced).
The Destroy Gundam is most notorious in that when the emotionally-unbalanced Stella piloted the beast, it absolutely devastated Berlin and even when stopped, the machine was still largely intact. When they start mass producing it, they drop like flies. Granted, ZAFT did introduce new prototype suits, but you'd think they'd at least be able to match the original's damage.
Somewhat justified in that the Destroy Gundam's greatest strength wasn't its firepower (though itwasquiteimpressive), but its ability to reflect back beam weapons. When they reappeared in later episodes, ZAFT knew that the way to deal with Destroy Gundams was to have your fastest units charge in and carve them up with swords, while the rest of your forces held off the escorts. And ZAFT had fielded a pair of very fast new Gundams after Berlin. Most likely, up to Berlin the Aile Strike Gundam (itself a unique Super Prototype]] was the only machine they had that was fast enough to pull this tactic off.
In the Alien series, some xenomorphs are victims of this trope. A warrior is the main villain of the first movie, while in the film Aliens, the warriors are demoted to Elite Mooks.
In Star Wars Episode II: Attack Of The Clones, during the chase scene with Obi-Wan and Jango in the asteroid field, Jango launches a missile at Obi-Wan's ship, which is treated like a terrifying, nearly inescapable weapon. Obi-Wan is chased for a good minute before he gets lucky and manages to destroy it by tricking it to fly between two huge asteroids. In Revenge of the Sith, Anakin and Obi-Wan are flying in a space battle and FOUR of those same missiles come after them, and they act like are no big deal and destroy them in about fifteen seconds just by SPINNING.
In "Trial Under Fire" (the Mech Warrior 3 novelization), the team's first encounter with a 100 ton Annihilator is described as an epic battle. Mid book, it is stated it "no longer held any special terror for" the main character, and towards the end, the lance destroys them two or three at a time.
In Power Rangers SPD, there was a three-tier Mook system: Silver ones cannon fodder, blue ones smarter and tougher, gold ones mega-badass. At first. The first Bluehead was actually the series' first Monster of the Week. They became much easier to deal with afterward being more Elite Mooks (though it never got to the point where the Rangers could beat them unmorphed.) Then there were the Orangeheads. The first one gave the Rangers a lot of trouble, clearly outclassing the two it fought at first and requiring the whole team to go all-out. The second one was powerful against but eventually fell to two Rangers. It was a while to the next one, but from then on, they were nothing special. Of course, as grunts of all tiers were summoned in ever greater numbers, it seems they just fell to the Law Of Conservation Of Ninjutsu. One Orangehead will always be worse than four.
The next series also had a multi-tier grunt system. Hidiacs were the standard grunts, with Styxoids treated as being fairly elite, and on a couple occasions, one Styxoid would lead a bunch of Hidiacs, a la Blueheads leading standard Krybots. By the end of the series it was common to see the two groups treated no differently; grunt fights had a mixed back of Hidiacs and Styxoids just to make the scene more flavorful.
Whether this trope applies in a role-playing game depends heavily on how steep the power curve between new and experienced characters is. In a game where high-level characters are far, far more powerful, such as Dungeons & Dragons, this trope almost always will apply. A monster is used as a boss for a third level party won't make a seventh level party sweat and will make a fifteenth level laugh.
Dungeons & Dragons 4th Edition actually invokes this with the Elite and Minion monsters. Elites are sort of mini-bosses, being twice as tough as normal monsters, while Minions are weak and only have 1 HP. You could fight a level 4 Elite orc enemy, then later fight a level 9 normal orc who is very similar to the Elite you fought 5 levels early. This could be taken further by fighting a level 14 Minion orc who looks like the two previous enemies. In addition, you could downgrade a Solo Monster (IE boss monster) to a higher level Elite.