In general, a lot of superhero shows will inflict at least a mild form of this on the Rogues Gallery, as villains who carried whole episodes by themselves in early appearances are reduced to Villain Team-Up or Legion of Doom stories in later ones, and non-speaking cameos in later ones still (usually with a hefty does of Villain Decay in the process). See below for specific instances.
Cousin Itt only showed up in three episodes of the 1973 The Addams Family cartoon albeit appearing in the opening.
Most, if not all of the supporting characters from American Dad! have fallen victim to this at some point, mainly when the show began phasing out it's political edge around the end of the fourth season/beginning of the fifth:
Greg Corbin & Terry Bates were probably the only recurring characters from the show's early days to still keep appearing regularly after the show's political angle was all but removed. But even they eventually found themselves falling victim to this, which was after the show moved from FOX to TBS when Mike Barker (voice of Terry, co-creator and former showrunner) left the series allegedly due to creative differences. Because of this, Terry was eventually Put on a Bus (but not before temporarily becoming The Voiceless) while Greg's appearances have severely diminished and are often very brief.
Tony Toponi and Bridget get demoted to background characters with a couple of blink-and-you-miss-it cameos in An American Tail: Fievel Goes West after having played a pretty large role in the first film.
The Avengers: Earth's Mightiest Heroes!: Hank Pym went from one of the core team members in the first season to a minor character that only appeared in about three or four episodes of the second (to be fair, Hank had to deal with personal problems).
The Riddler rarely appeared much, but what episodes he did get were quite excellent and put a refreshing spin on the character. When the sequel series The New Batman Adventures rolled over, however, ol' Eddie got the shaft, despite getting an entirely new character design more faithful to Frank Gorshin's take on the character. You know that things are bad when the new Riddler's biggest role was in a Superman episode. Paul Dini admitted in the "Art of..." book that they didn't use the Riddler often because his gimmick of using riddles was pretty hard to pull off in an action cartoon such as theirs. On the other hand, he had some truly spectacular moments in The Batman Adventures, set in the same universe.
Summer Gleeson (who was basically Vicki Vale in everything but name) appeared in a pretty large number of episodes in the first two seasons, as well as the Mask of the Phantasm and SubZero movies. When the show was Retooled as The New Batman Adventures for its final season, she made only two appearances, one of which was a non-speaking cameo.
Dana Tan in the second and third seasons, with the introduction of Max. Went from being Terry's main "normal person" confidante and having a scene in almost every episode to mostly being a background presence who got tossed a few lines every now and then, except for the one episode where she got to be a Damsel in Distress. Notably, she was (and remained) the OfficialLove Interest.
Terry's little brother and mother, Matt and Mary McGinnis, while not as prominent as Dana, also had less and less screentime as the series continued.
Virtually every villain (except for Joker, Penguin, and arguably Hugo Strange) got hit with this in later seasons - some (like Killer Croc) being reduced to bit players right after their debut episodes. Arguably reached its nadir in "Rumors", where the new Villain of the Week captures the entire rogues gallery without much trouble, and when they later escape and attack Batman and Robin en masse, all of them get taken out in a matter of minutes.
The 2006 revival of Biker Mice from Mars did this to the original series' main villains Lawrence Limburger, Dr. Karbunkle, and Greasepit, since the main antagonists were now a race of Cat-like aliens called Catatonians as well as a Corrupt Corporate Executive named Ronaldo Rump. While Limburger, Karbunkle, and Greasepit do appear in some episodes, their attempts at upstaging the Catatonians and re-establishing themselves as the greatest threat to the Biker Mice always fail miserably in the end.
Vlada Veramirovich in the webisodes of The Critic.
In The Fantastic Four (1978), Medusa is the leader of The Inhumans instead of Black Bolt. Black Bolt only gets a brief cameo where he's seen flying over Attilan, and doesn't have any real role in the episode.
In the original series of Fireman Sam, Trevor Evans the bus driver was a regular, prolific character who sometimes helped Sam by acting as a part time firefighter. In the newer series he's lucky if he gets a short appearance with a single line of dialogue.
Arlene, despite being present in the intro sequence of The Garfield Show, shows up only in a handful of first-season episodes and is thus far absent in the second season. She has made some more appearances since then, having larger roles in some of the specials but it still doesn't help that other recurring characters have made more appearances than her.
In Hey Arnold!'s movie, Phoebe Hyerdahl only gets one line, and the only memorable thing she does in it is run into some newsguy's camera. For that matter, none of the minor recurring characters, except Brainy, get any credit. Fortunately, The Jungle Movie seeks to address this and give the other characters an ample amount of screen-time along with the main characters. According to this Wikia article made before the movie was Un-Cancelled, the much-ignored Nadine will finally receive significant screentime in it.
Betty Ross, who had major focus in the first season of The Incredible Hulk, only appeared in certain episodes in season 2. Also, Doc Samson and Rick Jones were demoted even more after season 2's premiere.
The Big 7 in some extent suffers of this in the last season. Despite being the main characters, they only appear in less than half of the final thirteen episodes, with some episodes ("Patriot Act," "Grudge Match" and "Alive!") focusing on recurring characters instead.
Spryte. In later episodes of The Legend of Zelda, she is sometimes completely absent, with no explanation at all. One of the episodes that don't feature her explains her absence by saying that she's on vacation.
The short-lived character Beans the Cat was demoted when the spotlight shifted to his stuttering sidekick Porky Pig, and Beans and the rest of his co-stars soon faded into oblivion.
Before Daffy Duck became his sidekick Porky's best friend was a goat named Gabby who appeared in four cartoons, and he also had a girlfriend Petunia who also vanished after a couple of years (although she remained prominent in the comic books).
Porky Pig himself started to slowly drift from the spotlight. By the fifties his solo series was ended, with him working near solely as a sidekick for more abrasive characters such as Daffy, and by the sixties he disappeared with the exception of a couple of shorts. He appears more frequently in modern projects, if still mostly as a supporting character only.
Elmer Fudd was one of the first Looney Tunes characters ever created and established as the original Arch-Enemy of Bugs Bunny. In this show, you could count the episodes he is in on one hand.
Sylvester and Tweety.
If we speak of in-series, Marvin the Martian. In the first season he's one of Daffy's friends and provides to the plot in a few episodes. In season 2 he doesn't speak at all and is lucky if he even makes an appearance.
GIL-9000 and robot players in general in the Mutant League cartoon. Despite robots being one of the three races in both Mutant League Football and Mutant League Hockey, they appear about once, as training dummies at that. GIL-9000 in particular gets the worst of this; he's the only "star player" (with a portrait and unique quotes) from Mutant League Hockey to not appear on the show.
In The New Pink Panther Show, after the episode A Nut at the Opera, the panther did not appear for a full four episodes, excluding Driving Mr. Pink.
When the Winx ClubSpin-OffPop Pixie was announced, one of the major concerns about the series was the apparent disappearance of Digit (Tecna's pixie). She didn't appear in any official art at all, and the first that anyone saw of her... was when she appeared in the opening titles. (The other bonded pixies? All present and accounted for.) When Digit did show up, she was revealed to be male in this continuity.
Many major characters from Recess appear in the movie...but most of them appear very briefly, with absolutely no lines. Notably, TJ's arch-rival Lawson shows up for two seconds and gives a mere thumbs up with no dialogue, but he's listed as a speaking role in the credits. Erik Von Detten's other character, Captain Brad (who appeared in one episode of the show), has numerous lines. Some major recurring characters, like Cornchip Girl, don't show up at all.
In Recess: Taking the Fifth Grade, Miss Grotke only has two lines in the entire movie. Justified, as the main kids were in the fifth grade and weren't in her class anymore.
While Tails was pretty prominent in Sonic 2 and Adventures of Sonic the Hedgehog, he was reduced to minor status here. However, he did gradually get some more screen-time and the odd episode that gave him focus (particularly towards the end), but for the most part, several of the invented characters for the show were more in the "supporting cast" category than Tails was. Word of God is that, had the series gotten a third season, Tails would've gotten more focus and his role would've gotten closer to that of the games.
Bunnie and Rotor are also fairly minor characters in the second season, with Sally and Antoine played as Sonic's more prominent allies.
Despite being a member of Titans East in Teen Titans, Bumblebee is all but ignored in Teen Titans Go!. She didn't even appear until several years into the show (and in a non-speaking role at that).
It is very noticeable when this happens in The Simpsons, in large part because the show tends to exclusively use “real” characters in crowd scenes, making it easy to notice who’s playing the extra role a lot these days.
Herman is one of the main characters in the Season 1 episode "Bart the General" but his active role in the series essentially ends after this.
Reporter Dave Shutton, who helps Bart and Lisa derail Mr. Burn's candidacy for governor in the season 2 episode "Two Cars in Every Garage and Three Eyes on Every Fish" seems like he is being positioned as a major journalist character in the show, but beyond a few small appearances in subsequent episodes, he is quickly sidelined as Kent Brockman becomes the major reporter in Springfield (ironically, as Brockman himself was originally intended to be a The Unseen).
The show was originally supposed to feature far more child characters than it does now, with more classmates and friends for Bart and Lisa. Early episodes introduce many child characters, including Lewis, Janey, Richard, Nelson's "weasels," and Wendell who never wind up doing much of note in subsequent seasons, instead mostly becoming filler characters for Springfield Elementary scenes. Lampshaded in a few episodes, such as "Das Bus," when Bart can't remember who's Lewis and who's Wendell.
Homer’s coworker Charlie, who was briefly treated as a third to the Lenny and Carl duo, now mainly functions as a prop in Power Plant scenes.
Princess Kashmir, the stripper from "Homer's Night Out" in Season 1, weirdly enough, also seems to be something of an example — she featured prominently in early merchandise featuring the “extended cast” but is now entirely crowd filler, if that.
Happens to Tombstone after the Green Goblin arc of season 1 and up until the episode "Gangland". In the first episode of the series he sends Enforcers to destroy Spider-Man and is set up as New York's most powerful crime lord. He becomes the Big Bad during episodes 4-6, ordering Norman Osborn to create supervillains to distract Spider-Man and is set up as one of Spidey's arch-enemies. He also plays a big role in the Green Goblin arc (7-9), fighting against Green Goblin and teaming up with Spider-Man to stop him. However after that, he becomes a secondary character and in the Symbiote arc he only appears briefly in beginning of episodes 12 and 13 and only accepting job offers and nothing more. Thus, Symbiote/Venom replaces him as Big Bad of Season 1. In the first half of season 2 (Master Planner and Venom arcs) he doesn't even appear and is only mentioned in "First Steps". In the Gang War arc, while he is set up as one of the crime lords fighting for control over New York, he doesn't appear in Accomplices and appears in the beginning of "Probable Cause". However in "Gangland" he returns as one of the big bads and fights against Doc Ock, Silvermane, and Spidey. In the final episodes he doesn't get mentioned at all.
Same thing happens to Norman Osborn (Green Goblin) in episodes with Venom.
Many main or reccuring characters only receive cameos or minimum dialogue in The Movie, with the rest of the focus being on SpongeBob and Patrick and Plankton. Midway through, Mr. Krabs spends the entire movie frozen, Squidward's dialogue is rather limited in contrast with his speaking roles on the show, and Sandy, Mrs. Puff, Pearl, and Gary (not that he has much to say anyway) only appear in cameos.
As far as the series itself goes, pretty much every character that isn't SpongeBob, Patrick, Squidward, Mr. Krabs, or Plankton.
Frieda in Static Shock, especially in comparison to her role in the comic series. She starts the show as Virgil's primary love interest, and is actually the first "civilian" to speak to Static, but after Daisy is introduced, she quickly becomes superfluous to both the plot of the show and the relationships between the characters.
In the 2003/2007 version of Strawberry Shortcake, Honey Pie Pony's final appearance was among the audience in It Takes Talent / Playing To Beat The Band, with no lines and no plot. She was then shoved onto the bus.
King Koopa, of all characters, got this treatment in the Super Mario World cartoon. After appearing in nearly every episode of the first twoanimated adaptations, he had main roles in virtually half, and four episodes did not feature him at all!
The Koopa Kids suffered even worse from this fate after being the prominent minions in The Adventures of Super Mario Bros. 3. Their appearances in Super Mario World became limited to the point where Kootie Pie, Kooky, Hip and Hop each had speaking roles in only one episode.
Tweety became less active in later Sylvester the Cat and Tweety Bird shorts, the main bulk of which revolved more around a bodyguard or alternate adversary guarding him from Sylvester (usually Granny or Hector).
In the original Mirage Ninja Turtles comics, vigilante Casey Jones is a good friend of the turtles and one of the most prominent members of their supporting cast. In this cartoon, well, if you remember him at all it's as that loon who showed up in a few episodes wanting to break things.
The Triceratons, one of the most iconic recurring Turtles villains who appeared in numerous media including the original comic, the games, the toyline, and the 2003 cartoon, only appeared in one of the later episodes of the 1987 show.
After Carter showed up, April was relegated to sitting in her apartment and researching stuff on the Internet for all of Season 9 and the first few episodes of Season 10. Fortunately, she started taking a more active role in the final few episodes once Carter was written out.
In Season 5 of the second Teenage Mutant Ninja Turtles cartoon, Casey and April really fell into this, largely a result of the season's sharply reduced episode count; it did not help that half those episodes were spent outside New York. They grew even less prominent in the Fast ForwardRetool, since they did not accompany the turtles into the year 2105, and could only appear as cameos or in flashbacks. Back to the Sewer reversed the trend, however, giving them a season-long arc in which they prepared to get married.
Thomas and Friends: The show does this regularly, as a result of having Loads and Loads of Characters and the tendency to introduce several new characters every season. As a general rule, the new characters get a couple of seasons in the spotlight at most before demotion:
Edward in seasons 4-6. Given a bit more screentime later on, at the cost of his classic personality.
Toby's been getting the worst it lately. In Hero Of The Rails, not only does he get only one line, not only one word, but one Syllable. He was completely absent in Tale Of The Brave.
Captain has done nothing of value since his introduction in Misty Island Rescue.
While he still gets the odd spotlight episode, Hiro’s appearances are more sparse.
Duck. Before he disappeared altogether, from when the show started deviating from the books. His last starring role prior to "The Thomas Way" was Season 4's "Fish". Subverted following his return in Season Seventeen, where he has been getting regular key appearances.
Scruff was relegated to the background for two seasons following his debut, but got some spotlight once season 17 aired.
Rosie made regular appearances in the model seasons. Come the CGI transition, she has had only one spotlight episode, being reduced to quick cameos otherwise.
In season 4, Duke was a major focus of the first four episodes. For the rest of the season, he was relegated to the background. In addition, he didn't have a single role at all from that season onwards. Even now, he still hasn't made an appearance.
After their episode, Mighty Mac were quickly relegated to the background with only a few minor speaking roles.
In recent years Bertie the Bus’s appearances have become scarce, when he does show up, he's usually either avoiding an accident, or in need of rescue after getting stuck or breaking down.
Snarf plays a much smaller role in ThunderCats 2011. Sometimes halfway through the episode you find yourself remembering he's still in this version and wondering where he is.
It seems like Gogo Dodo was originally intended to be a principal character in Tiny Toon Adventures; he gets a fair bit of attention in the Series Bible, he's one of the characters to be mentioned in the theme song, and he plays fairly large roles in the Origins Episode and several episodes early in Season One ("Her Wacky Highness", "Sawdust and Toonsil", etc.). In later episodes he's a very minor character.
Transformers is somewhat the reverse of many other examples on this page, in that a number of characters appear only in the toyline. That said, there's plenty of straight examples,
As an example, Jazz, who was a major character in the first two seasons of G1, had a supporting role in the movie, but when Scatman Crothers died shortly afterwards, Jazz's later appearances were non-speaking cameos.
Many G1 characters survived the events of The Movie but were gently pushed to the background for marketing purposes. Hasbro wanted the series to showcase their 1986 additions to the toyline. Most of the car-based autobots (such as Cliffjumper, Prowl, and Bluestreak) were rarely seen in the 1986-87 season.
From the latter half of Transformers Prime Season 2 onward, Raf suffered a rather diminished role compared to that of Jack, Miko, and Fowler. This is also seen in him getting the least amount of Character Development throughout the series. A pity considering he made a great partner for Ratchet and had a strong showing in the first season.
Scooby-Doo himself, in most cases, and sometimes Shaggy in What's New, Scooby-Doo?. Typically the show would shift focus onto Fred, Daphne and Velma, but one major exception was the episode '"Camp Comeoniniwannascareya", which did not feature Fred, Velma or Daphne at all; it just featured Shaggy and Scooby-Doo as the main characters.
Roxy is set to appear in only seven of the twenty six episodes of season 5. It's been finally confirmed that she's NOT a Winx (she's completely absent from the season 5 opening).
The pixies are often subject to this. In season 2, they served as fairly important characters to the plot. In season 3, barring their A Day In The Lime Light episode, they serve mainly as comic relief alongside Kiko. In season 4, the are reduced to one episode, and are completely absent in season 5. They return in season 6, but as comic relief again, and two of their members are replaced with Canon Immigrant from Pop Pixie.
Mirta. Some of the events from season 1 revolving around her character, culminating in leaving Cloudtower for Alfea, suggested that she was going to be the sixth Winx girl. For some unexplained reason and despite having a good friendship with all of them (especially Flora), she never became part of the group. She does manage to get a cameo multiple times, establishing she's still around.
In the animated series of W.I.T.C.H., The Oracle has a much smaller role and doesn't even meet the Guardians face-to-face until the second season, probably because Yan Lin is at hand to explain things.
Happens to Storm, of all people, in Wolverine and the X-Men. While a major character in the comics, one of the most recognizable members of the X-Men, and being in the main cast of nearly all the previous animated adaptations, here shoved in the background to give Wolvie a bigger role. In fact, there's no real reason for her presence at all: she doesn't factor into any of the storylines, and the episode where she rejoins the team has nothing to do with the overall plot. In the episodes in which he takes part in battles, she appears, makes a major power play, and is knocked out almost immediately. The writers even acknowledge that she's too powerful to work into their universe properly. It's possible that the writers only threw her in because viewers are used to seeing her. To say nothing of Jean, who was reduced to nothing more than a plot device.
Wally Walrus from Woody Woodpecker was brought onto the "underdog" cast section when other characters, which can be obviously referred to as bad guys, like Buzz, Dooley and Dirty McNasty came up. At least not until the New Woody Woodpecker Show.
Wuya started out as the main villain and the most considerable threat the monks face, especially when she regained her physical form the first time. While she is still present in both series, her role as the main threat was replaced by Chase Young in both Showdown and Chronicles.
Clay got this treatment in both series; his calm and rather silent personality made it easy for him to be overlooked by his friends' more colorful ones. He received the least amount of attention in terms of spotlight episodes and the least amount of development compared to the other monks.
Raimundo received this treatment in the second series, which omits his entire character arc of becoming the leader the first series ended with. Here a single episode focused on him and even then, Omi took up most of the plot, not to mention he was the only monk not challenged to an individual showdown. He and Clay were essentially those two guys in the background for most of Chronicles.
As a whole, this happened to the monks in the second series, as most of the attention is focused on Omi and Ping Pong. It wasn't common to see Clay, Kimiko, and Raimundo on the sidelines or easily taken out before the former pair came along to do the real fighting.
Due to the or iginal comic having Loads and Loads of Characters, it was inevitable this would happen in X-Men. Two notable X-Men, Colossus and Nightcrawler, were relegated to cameos and guest appearances. This was because they were a part of Excalibur in the comics at the time, as well as Marvel's desire to focus on the more current 1990s members of the team. They at least fared better than poor Kitty Pryde, who didn't appear at all.
X-Men: Evolution has Spyke, a major character in seasons one and two; in season three, he up and joins the Morlocks. Gets an episode here or there, but he's mostly gone. This wasn't necessarily a bad thing, because he was Rescued from the Scrappy Heap when he returned. Storm, who happens to be his aunt, also gets this very badly. Despite always having been a major player in the X-Men franchise, she receives very little focus in this series and only has one episode centered around her during the entire run of the show.
The entire Justice League, since the series focuses on the sidekicks of the team. Cartoon Network even touted the show this way in early promos, counting on the audiences familiar with the Justice League cartoon to cause shock when the protagonists were revealed to be the children.
Red Tornado's conspicuously absent in the second season (likely because Nightwing and the others have replaced him in the mentor role), and has only had one non-speaking appearance in "War".
Zatanna and Rocket in season 2, both of whom graduated to the Justice League. Ironically, both of them were the last to join the team in the first season.
Secret, when compared to the comic. In that, she's a member of the team. In the show, she is present in only one episode and is limited to Pokémon Speak.
The entire cast of Aladdin, including the titular character, suffered this fate as the later merchandise based on the film now focus on Jasmine. Considering that The Disney Princess line of merchandising is quite profitable and has become in an odd way, it's OWN spinoff from the various Disney films that each princess comes from, this may not be the case with Jasmine—or perhaps, the fate of any character associated with a Disney Princess.
If your only exposure to the franchise is the coloring books, you probably don't know that there's a male fairy. According to the web game, he's a "sparrow man" despite obviously being a fairy and grouped as such before. He still gets no attention.
Pete was one of the main characters on Goof Troop, then was demoted to running a major subplot that took up about a third or fourth of A Goofy Movie, only to get two scenes and one brief flashback to an earlier scene of screentime in the sequel and be rendered irrelevant by the end.
Minnie Mouse started out as Mickey's co-star for most of the black and white shorts, but she started to appear less and less once Donald began appearing in Mickey shorts and he, Goofy, and Pluto got more screentime. Once Donald, Goofy, and Pluto each got their own series of shorts, even Mickey himself became the victim of this.
Bo Peep in the Toy Story series, while never a big character, has suffered this. In the first Toy Story, she at least gets several scenes that develop her relationship with Woody and is present throughout the story, along with scenes where she is the main focus as she wonders why he and Buzz have gone. In Toy Story 2, she is still present, but only for the first 20 minutes, but still takes up the (offscreen) job of managing the other toys until Woody, Buzz and the rest of the crew return at the end of the film. In Toy Story 3, however, she is only present (sans voice) in Andy's home movie, and soon afterward is said to have been sold at a garage sale. However, she will reappear in Toy Story 4.
When Josie and the Pussycats wound up launched into outer space, Sebastian the cat became this after the band adopted Bleep, a little fuzzy alien creature.
Care Bears: Adventures in Care-a-Lot promptly stripped Tenderheart Bear off his status as the leader of the Care Bears and gave it to Cheer Bear. Tenderheart even received the least toys during that era of the franchise. Understandably this was one of the contributing factors to the broken base of the show. Another character that was demoted to extra was Good Luck Bear, who appeared only in background in the show, which somehow led some fans to hate Oopsy Bear because they somehow believed that Oopsy replaced Good Luck.
Brittney was another classmate of Star's that appeared regularly at the show's beginning and nearly disappeared in the second season—she only had two, brief appearances in the foreground, one of them non-speaking. She was likewise removed from the last shot of the opening.
Django was originally set up like he would be a part of the boys' main group, but after one Day in the Limelight episode he was pretty much relegated to background shots.
Also all of the other kids who rode on the boys' rollercoaster in the first episode. Did you even know they have canon names? This is lampshaded in "Rollercoaster: The Musical"—they take Phineas and Ferb's poster like in the first episode, only for Buford to snatch it away, calling them "lousy extras." He, Baljeet and the Fireside Girls take their place on the rollercoaster.