Pokémon, Sailor Moon and others in theirgenres are well-known for this. This trope is very common in some varieties of anime, and in anime it tends to take an egregious form that, after watching a few episodes, causes the audience to start asking uncomfortable questions like "Well, why don't the bad guys attack all at once instead of one at a time?" Writers usually stoop to handwaving if they deal with the question at all.
Sailor Moon is the most famous of this, with the monsters of the week - at least 80% of the time - also being Monogender Monsters, females in this case. Also, with only one exception (Cienicienta, but not because she's strong, smart or otherwise special, just because the authors wanted to make Usagi's birthday-episode into a two-parter)none of the monsters ever survive the episode they were introduced in. While in some cases the monsters are mindless beasts (Cardians), other times they are shown to be intelligent beings capable of emotion - some not being outright evil even -, which makes it kind of odd how the Sailor Senshi never show any mercy to any of them.
Both parodied and played straight in Magical Project S, which has Pixy Misa summoning a new "Love-Love Monster" in half of the episodes. The show and its characters are quite aware of both the futility of these creations (as the incantation of "Calling Mistakes" suggests) and their formulaic nature (in an episode where Misa introduces a small army of them, Sammy dryly says "I've seen all those already").
Pokémon is well-known for this - some Pokémon get to be the Monster of the Weekmultiple times. Within the first 24 episodes, Gastly was monster of the week twice: when he was impersonating a statuified woman, and as part of the Lavender Town episode with its evolutions.
Fist of the North Star's Kenshiro regularly faced off against villains of the week, often with some weird Nanto or Hokuto derived power that he had to overcome, moreso in the anime than in the manga, and the series varied between these and genuine story arcs.
Mazinger Z and its sequels -Great Mazinger and UFO Robo Grendizer- are considered the 1970s paragon in spite of they actually subverted the trope. Dr. Hell often sent two, three or even more Mechanical Beasts at a time as well. And the original manga averted it completely. The monsters gang up on Mazinger more often than not. It also was subverted in Mazinkaiser, where Dr. Hell sends a large number of monsters at once against Z and Great Mazinger, and wins.
Samurai Pizza Cats. Lampshaded in one episode, where the Big Cheese introduced the robot menace he'd prepared for this episode with "Monster of the week, please enter and sign in."
Nightmare in Kirby of the Stars would provide King Dedede with a new monster with which to try to kill Kirby every episode. Naturally, Dedede is just too cheap to buy more than one at any one time. He did go into debt buying them. Nightmare actually had to send a monster to collect the debt without him realizing (At first). Still was defeated though.
Fullmetal Alchemist played this for the first volume before going into the main plot (which it would keep through the entire series); interestingly it was still only one of the chapters of the first volume that didn't affect the story in any way.
Likewise, some early episodes of the 2003 anime adaptation had a version of this: if there's a plot important character in the episode we have not seen before, he is probably the Villain Of The Week. The main exception to this rule is Rose. 'Course, a fair share of these episodes turned out to be important to the plot later.
Played with and used straight by RahXephon. The Dolems mainly show up on a one-a-week basis, although some of them survive their initial appearance and go on to reappear later.
Jo Jos Bizarre Adventure arguably boils down to this... except it's "villain for the next month and a half", due to the length of the fights.
The series didn't adopt this format until Part 3 began and Stands were introduced, as Parts 1 & 2 were arc based. Interestingly enough, this format helped the popularity of the series, as the fights were unique and the Stand users diverse.
For about the first half of Speed Grapher, Suietengu's plan to recapture Kagura is to have his henchmen sic a different Euphoric on Saiga. They never live more than two episodes after being introduced.
Early chapters of Yu-Gi-Oh! generally featured a "Bully Of The Week." His role was typically to scam or beat up Yugi's friends, at which point Yugi would challenge him to a Cooking Duel or the local equivalent. Most notably, The Rival Kaiba started out like this, butthen...
The anime based on the manga, and its spin-offs, have a Duelist of the Week who pops up with a new deck gimmick and quirky personality to challenge the hero. With very, very few exceptions, these characters will be defeated in a single episode and will never appear again. If they're lucky, they'll get a two-part episode before they vanish.
Every season of Digimon starts out this way as the new characters learn the ropes and the viewers learn the new characters (and in some seasons, new universe.)
In the case of Digimon Tamers this is generally held to be what killed the show's american ratings as a true big bad was not introduced until the trip to the digiworld 24 episodes in. Save maybe season 4 where it states that most all monsters are sub races of digimon, save a few, from the start.
The anime Dai-Guardhangs a lampshade on this one by having scientists predict that the conditions necessary for the alien giant monster invaders to appear will repeat themselves roughly once every week.
Figure17 plays this completely straight, although there is strong continuity as well. By the end of the series the monsters don't even look different from each other- they just get slightly upgraded powers.
This does become less prominent as the plot goes on, however, as emphasis shifts toward Tsubasa and Hikaru's relationship, with some episodes not featuring a Maguar at all, and others being dedicated to particularly large and important, multi-episode fights.
Witch Hunter Robin got a new witch every week for the first half. Then things changed rather abruptly...
Hell Girl sends one soul to Hell almost every week.
Mobile Suit Gundam, while extremely arc-based, still managed to introduce a lot of new enemy mobile suits in a monster-of-the-week fashion.
G Gundam is THE best example in Gundam, because this was the entire point behind the series, to draw on the Monster of the Week fan base, or more specifically the robot of the week fanbase, because that was how most robot shows were done prior to Gundam.
SD Gundam Force did something like this during early on. Basically, the Dark Axis' Quirky Mini Boss Squad would use a [[Mindcontrol Control Horn]] on a robot in Neotopia, such as a swan ferry or a train, turning it from a helpful mobile citizen into a dangerous menace, and it would be the Gundam Force's job to destroy the Horn. According to the Zako Zako Hour, the Dark Axis (Or at least Zapper's Squad) were not very good at making new weapons, so they have steal everything. This stopped after episode 9 as the Dark Axis began sending in better warriors to attack Neotopia.
Played straight in GaoGaiGar with the Zonders, though taking things in canon time passage it could more likely be considered the "Monster-of-the-half-a-week".
Played straight by Genesis of Aquarion, though at first the monsters were just regular Cherubim Soldiers with some kind of new ability that the team had to find a way to overcome by using lessons from earlier in the episode to unlock a new attack.
Bleach started out like this, with Ichigo fighting a different hollow each chapter. Though after Rukia got taken back to Soul Society, it became more Story Arc focused.
Even during Story Arcs, his duels with Lieutenants, Captains, Arrancars, etc, all count as part of this trope. Duel of the Week, perhaps?
InuYasha was this for nearly every episode outside the last anime story arc. Once the manga got past the point of the anime ending, it changed up a little bit.
Even before that she collected some Jewel Seeds off-screen and several per episode
Often overlooked, but Neon Genesis Evangelion also started like this. From the 13th angel's attack on though, even though each monster's still gone at the end of the episode that introduced it, the mental scars its attack leaves behind on the main characters remain...and build up over time. Evangelion's cast is on its last legs by the time the 17th angel kicks the bucket. CueEnd of Evangelion.
The Getter Robo series did this a lot. The original, G, Go, and to an extent New all used this trope. Even the crossover movies were Monster of the Week.
In Sonic X, the first 26 episodes of the first series had Dr. Eggman's randomly-deployed robots, each one with an E-(insert number here) as their serial number, and the first 11 episodes of the first half of the second series had random Metarex encountered by Sonic and co. along their journey to save the universe from the Metarex.
Kekkaishi follows this trope, with a strange new Ayakashi or two attacking the Karasumori site every night. But it's justified through the actual behavior of the ayakashi, the motives of more dangerous ones, and the steady plans of the Kokuboro.
Tengen Toppa Gurren Lagann plays this straight. In the first episode, the Lagann is introduced as well as a minor enemy. Next episode introduces some more mecha, including the Gunzan (later Gurren). Third episode introduces the first actually recurring villain, who's more of an anti-hero. By episode 6, the show actually starts straying becoming more serialized rather than episodic, but maintains its Monster of the Week standard until episode 15, where the MOTW is actually the first Big Bad. The second half reversed this, by having the good guys introduce more and more powerful mecha to kick the enemy's ass, most notably after Team Dai-Gurren goes to space.
With the exception of the series' recurring antagonist Vicious, most Cowboy Bebop episodes centered around a single villain or group of villains that was never heard from again after the end of the episode (some were two-parters).
This is the basic structure of Rinne so far, albeit longer than most of the examples on this page — most cases take two or three chapters to solve.
For a while in Popcorn Avatar, this is how many of the Asura and their avatars appear in front of Kurando.
In the old The Dandy comic strip, Jack Silver, the villianous Captain Zapp had a device known as a Duplicator, which could create a living, breathing copy of any picture that was fed into it. Every week, he would use the strange creatures the machine produced to commit crimes, before being stopped by Jack Silver and his gadget of the week.
Many comics tend to have a story with a one-shot villain every now and then. It would be easier to list comic books and comic strips that DON'T utilize the Monster of the Week trope.
Most chapters of My Little Unicorn deal with the good guys blowing up Titan's recently created monster.
The Doc Savage novels are always this except one because Doc is so good at what he does (lobotomies).
In the books of the Star Wars Expanded Universe, earlier-written ones in particular, the vast majority of villains are only there for the book or trilogy, and books set later or earlier completely forget that these villains ever existed. Odd, considering that they tend to be Imperial forces. The exceptions are Aaron Allston's run on the X-Wing Series, which had the campaign against Warlord Zsinj; the Coruscant Nights trilogy, which had one-book guest appearances by Prince Xizor and Aurra Sing; and roughly anything Timothy Zahn writes.
In the Trixie Belden series, there's almost always a new villain in every book.
Live Action TV
The 4400: Just under half of the episodes were like this. Several episodes would focus on a specific person out of the forty-four hundred people who had disappeared and been returned (and, later on, people who had taken the Promicin shots handed out by Jordan Collier), what sort of supernatural power they had developed, and a problem they had created (either willingly or otherwise) that would be resolved by the end of the episode. As stated above, it could in this case perhaps be more accurately called something like 'Freak of the Week', as the people in focus weren't always deliberately antagonistic.
Agents Of SHIELD: The show starts off this way, though the Centipede organization and the mysterious Raina reoccur throughout various episodes.
Babylon 5: Had these from time to time, most often in the first two seasons, with Story Arc episodes mixed in and becoming more common as the show continued. By the third season, such episodes became very rare as the plot began to reach critical mass.
This show was originally supposed to be an edutainment program...until the Daleks showed up, whereupon it careened irreversibly into Monster of the Week territory.
Notably, the old series was made up of serials, usually three or four parts...making it more like monster of the month. Though, the new series follows this trope straight, while also including more Story Arcs.
They even lampshade this trope in "The Eleventh Hour", during Matt Smith's epic speech: "'cause you're not the first to have come here, oh, there have been so many!"
Farscape: Had monsters of the week interspersed with the Story Arc episodes throughout the series.
Fringe: Started out as primarily a monster of the week show, and it still has them, but now they're either in service of or as a distraction to the Myth Arc.
Grimm: The first half of the first series follows this to a tee. The second half of the series featured more character arcs and ongoing plotlines but it still largely stuck with one case every week, whether certain cases feeded into a previous one or not.
Haven: Revolves around Supernatural Mystery Disaster of the Week. The town seems to attract people who are "troubled" and have supernatural abilities.
Heroes: This show is normally entirely serialized, but volume three would often put the arc in the background for a one-off evolved human. Examples include the man who could create wormholes and the Haitian's brother.
Kolchak: The Night Stalker: Could be considered the ultimate archetype. It was, in fact, even mockingly dismissed by some as "Kolchak's Monster of the Week" when its transfer from a pair of movies to a TV series ended up not quite panning out.
Merlin: This was a big part of the first series, and the Big Bad only appeared in 4 of the 13 episodes. From series 2 onwards the writers concentrated more on a singular villain (Morgause, Morgana and Agravaine, though occasionally a one-off monster will appear for a Filler episode.
Primeval: Just what will come through the Anomaly this week? Gorgonopsid? Mammoth? Velociraptor? Future predator? Knight in Shining Armor? The show also has a Story Arc that ran parallel, with the heroes battling human villains while still handling the Monsters Of The Week, who filed both sides under "dinner".
The Prisoner: Had the No. 2 Of The Week, who tried the Scheme Of The Week to attempt to break No. 6. (There were a couple of returning No. 2's with new schemes.)
Red Dwarf: Goes this way after about the third series. To their credit, the crew is pretty genre-savvy about it, especially in Series 6. For example, Rimmer explains to one monster that everybody they'd met to that point has tried to kill them. It also swings the other direction in Series 7 & 8, having the storylines cover multiple episodes. (Although they are still self-contained.)
Has Meteor Freak of the Week, mutants created by kryptonite. Season 6 also gave us the Phantom Zone escapees-of-the-week. As the series has progressed, it has much more of a Story Arc, but of keeps the Monster of the Week format.
It also had an interesting variation starting in Season 4: Hero of the Week. Every season would have about two or three episodes where a superhero or two from the comics would guest star, run amuck, get on Clark's bad side, then ultimately they'd make their peace and help each other out before leaving in a manner that some could interpret as a Poorly Disguised Pilot. The CW's second superhero show Arrow seemed to play with this method when it introduced Huntress: At first she seemed like she was going to be a Hero Of The Week, then her second episode has her declare she'd rather practice revenge, not justice, then her third has her cross the Moral Event Horizon, turning her into a villain-of-the-week.
Space: 1999: One of the most common fan complaints about the second season was that it dropped the metaphysical and psychodrama aspects in favour of more Monster of the Week action-oriented stories.
In SF author David Gerrold's book about writing the episode "The Trouble With Tribbles", he recounts seeing the first episode broadcast, which featured a creature that sucked all of the salt out of people's bodies, thereby killing them. He hoped Star Trek wasn't going to turn out to be a Monster of the Week show, which ironically for him, it did.
Supernatural: Usually has actual monsters, more so in the first two seasons. This is balanced against the Myth Arc villains, who tend to be demons, rogue angels, and in season 7 Leviathans. In the later episodes the writers still bring in a Monster of the Week every once and a while for a breather.
Practically every series (and by extension, every Power Rangers series). There's no point in listing them all, just click the link to see them. Plenty of lampshading. "Just send them all" has in fact been tried before. If it's a small number the Rangers have a hell of a time with them. If it's a large number, Conservation of Ninjutsu kicks in and they go down as easily as Elite Mooks. Except for the time they had to kill Zordon...
Common in this genre, especially Kamen Rider, Super Sentai (and by extension, Power Rangers) and the Ultra Series. This isn't terribly surprising, as the action and fights are the main draw of these shows. Indeed, Super Sentai has multiple-stage monster of the week fights, culminating in a robot vs. daikaiju showdown.
Power Rangers may well be your average Westerner's introduction to the very concept.
The 51 Undead in Kamen Rider Blade are actually in conflict with one another to see which will be the dominant species on Earth (the human Undead was the winner of the last such competition, hence us). It can probably be assumed most of them are simply laying low and gathering their strength at the beginning of the series.
Parodied in one episode of Power Rangers Ninja Storm. Lothor tries to send six giant monsters at once against the heroes, only for his device to fail citing a "memory error". His general informs him that they did not pay for the memory upgrade, so they can only enlarge one monster at a time. Lothor curses at this complaining that as future ruler of the world "I need big monsters!'" and settles for enlarging one and making the rest fight while small.
Kamen Rider Den-O and Kamen Rider Double are a slight variation on this, as almost every episode is a two-parter (or more); therefore, almost every Monster of the Week actually lasts at least two weeks - and for the former show, that's not counting the ones that were just slightly rebranded and reused, or those revived to serve as the Big Bad's army in the Grand Finale.
Every show since Den-O has followed the "monster of the fortnight" formula, though OO Os did have a few Yummies that only lasted one episode.
Except for Metalder and 'Jiraiya every other Metal Hero (except the Rescue Police trilogy) show followed this, even Tokusou Robo Janperson'' has a "Cyborg of the Week" along with actual monsters.
Choujinki Metalder deserves special notice. In the first episode, every monster ever can be seen in the Big Bad's lair. There are four groups of monsters, each with its own general, and each has his or her own rank. Most are just in the background until it's their turn at being MOTW, but there are monsters who live to tell the tale and come back to fight another day multiple times, and some who rise to become major characters even if not "Victorious Saint" (general) rank.
Even an Adult-oriented Toku like Garo Has Monster of the Week episode, spliced in with Story Arc episodes, even on several occasions the MOTW was relevant to the arc.
Torchwood: ...At least, the first two series. After the successful switch to "mini-series focused on a single threat" of Children of Earth, RTD decided to drop the MOTW format altogether.
In the series, monsters of the week were produced by a chemical compound called "Mix Master" which, once released into the valley, randomly scrambled together the DNA of all living things except humans. This created monstrosities ranging from acid-shooting plants to giant shrimp.
And one of them was defeated by the resident monster, El Blanco.
The Troop: Nickelodeon's show is built around this trope.
Warehouse13: Revolves around the Artifact of the Week, which can range from purely a MacGuffin all the way up to an actual Monster.
The West Wing: Does this metaphorically, with Political Crisis Of The Week, caused by Idiot Politician Of The Week (in fact, many episodes are around one week long).
Unusually, while most fandoms considers MOTWs to be fillers, a large group of The X-Files fans considered the weekly monster episodes to be superior to the Myth Arc episodes, especially in later seasons... mostly because the latter were made up as they went along.
Although a different medium, episodic RPG campaigns also fall into this pattern, as gaming groups usually get together to play once a week.
The monthly Full Moon Shadows that the party fights in Persona 3 at first seem to fall into this category; however, later on, it is revealed that they are all actually fragments of a single Shadow, Death, who is the herald of Nyx, the one destined to bring about The End of the World as We Know It.
The true Monster of the Week was typically whichever Tartarus Boss your party was ready to fight on that night's run.
The Fire Emblem series uses this for bosses in the game chapters.
A literal example in The World Ends with You for the DS: each week of the Reaper's Game is presided over by a "game master". These are especially powerful reapers who, what do you know, transform into monstrous versions of themselves when you get to fight them.
The sets of 8 Robot masters in the Classic Mega Man series easily fall into this. The Game Boy spin-offs do this with the Mega Man Hunters (and Quint) and the Stardroids as well, while the sole Genesis game in the series (Wily Wars) had the Genesis Unit. Then, there's the fake major villains replacing Dr. Wily...
Taken up a notch in the Super Robot Wars franchise. Not only do the heroes have to deal with most (if not all) of the villains and monsters from their respective series (including those mentioned above), but there's also a new latest threat to stop on top of everything else.
Though a lot of these were the result of the machinations of one or more of the show's three Big Bads - Tombstone, Doc Ock, or Norman Osborn, rather than isolated encounters. What's really interesting is the show's justification for why there are so many supervillains running around: The Big Bads had them created to keep Spider-Man busy and thus unable to interfere with their standard criminal operations.
Megas XLR practically lives off this, along with a fair bit of lampshade hanging. "Cool. Lets go see what kinda monster I get to beat up this week!"
Lt. Barf: Captain, we are being hailed. I recommend we go to Red Alert! Captain Pinchhard: We haven't even met them! Isn't that a little premature? Lt. Barf: Every week we encounter aliens who try to destroy or take over the ship. It would save a lot of time if we assumed the worst now.
Martin Morning demonstrates this, with the odd twist of the protagonist being the new monster each episode.
Martin Mystery was this kind of show, with the characters being sent each week to investigate a mystery that always ended up being caused by a monster.
An episode of Batman: The Animated Series, "The Underdwellers", spotlighted a villain called the Sewer King who never appeared again. He was sufficiently creepy for a Batman villain, but it's just as well he never returned, since he was really only good for one story (that is, showcasing the evils of child slavery).
The same could be said of Baby Doll, as she only ever had 2 appearances, and was limited in both motive and ability compared to other, more menacing Batman villains.
Regular Show seems to be establishing itself as one of these; but you'd probably prefer to call it "Weird Crap of the Week."
Dr. Claw of Inspector Gadget had a new special MAD agent almost every week, who would never be seen again after the episode they appeared in. Gadget And The Gadgetinis did the same, but also had some one-time villains with no connection to M.A.D. or Dr. Claw whatsoever.
Godzilla: The Series has a new creatures of some sort in every episode. However, most of these monsters are actually pretty good, and they range from a petroleum-eating goo blob, to a swarm of ants big enough to uproot and carry a tree.
Dino Squad had a somewhat interesting variation on this. While it did have a central villain mutating normal animals into prehistoric creatures, and doing so was part of his master plan to take over the world, he only did so to gather scientific data to improve the mutation process. The mutants were almost never created to facilitate some other evil plan.