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.
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.
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.
In Danny Phantom, Valerie frequently appeared in Season One and Two, playing as the series' main Anti-Hero, then seemingly disappeared in Season Three until the second-to-last episode, then later made a small, speechless cameo in the Grand Finale. And her story arcwasn't even completed yet!
Luanne on King of the Hill was in nearly all the episodes for the first three seasons. When she moved into her own house midway through the fourth season she was quietly changed to a recurring character, often not appearing for weeks unless the episode was specifically focused on her. Later on in the series she was upgraded again and became a semi-regular, appearing in about half the episodes.
True also of Connie, whose appearances grew more and more infrequent after she broke up with Bobby.
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.
Goofy's son, Max appears to be focused more on than Goofy in most of Goofy's latest appearances, particularly in the 90s. While Goofy and Max get equal attention in Goof Troop, both "Goofy" movies and both Christmas movies, "Mickey's Once Upon A Christmas" and "Mickey's Twice Upon A Christmas", appear to be all about Max.
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. In the first film, she 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 the second film, she is still present, but only for the first 20 minutes, then 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, she is only present (sans voice) in Andy's home movie, and soon afterwards is said to have been sold at a garage sale.
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.
Most of the characters from the first few seasons, particularly Officer Barbrady & Dr. Mephisto, who were major characters in many episodes, and often had whole shows featuring them (both later vanished, and South Park even got a real police force).
Jimbo and Ned had major roles in many of the earlier episodes. Now, they rarely appear or speak anymore.
Tweek, who was a minor character prior to season six (where he replaced Kenny as the 4th member of the group), is now demoted to extra.
Kenny; after coming back from Killed Off for Real his role was greatly diminished and rarely speaks, although every couple of years since then he will get an episode dedicated to him. This however changed somewhat thanks to the Mysterion episodes. In fact, he's been in all the episodes of season 16 so far, having speaking roles in the majority.
Chef was showing up less and less (after being more or less the fourth most important character on the show) before he was killed off.
Pip, one of the most vocal and visible of the kids at South Park Elementary at the show's start, and got a whole episode to himself, got pushed away into the background as the show progressed, and has now vanished completely. Then he returned to have a mecha's foot dropped on him. This was apparently in part due to Parker and Stone being annoyed by fans constantly asking where Pip went, while they never truly liked the character (being that he was made as a jab at Charles Dickens' Pip and other "boring orphan" characters). Parker and Stone have also said that the introduction of Butters had a lot to do with Pip's eventual disappearance from the show, as he eventually took over Pip's role as the kid everybody picked on.
Cartman's mom was the parent that was seen the most often. Due to her serving Eric's every whim and being a total whore, she got around... until Stan's dad took the spot of 'Most Prominent Parent'.
Kyle's parents have gradually dropped out of sight, as well. Sheila was a major antagonist/Well-Intentioned Extremist in the early years, including the movie, and she's all but disappeared in the later seasons.
Stan's sister Shelley was established to be a sociopath who beat up her brother in often over-the-top ways, but now she's become a generic sister character, with her original personality showing once in a blue moon (see "Over Logging"). Later in "Broadway Bro Down", Shelley was given a subplot that attempted to show a softer side to her.
The Mayor appeared far less as time went on.
The boys themselves. More and more episodes seem to follow the formula of "Randy does something stupid, while Stan & Kyle get maybe one or two lines in the beginning".
Wendy, Ike, Fr. Maxi, and Mr. Garrison have scarcely appeared in later seasons.
In The Boondocks, Michael Caesar, who moves to the neighborhood shortly into the comic strip and is Huey's best friend (and, at times, the Straight Man to him), is completely absent from the television series. Huey gets this treatment in the middle of the second season, since his purpose is to express the creator's opinion on the situation at hand and remind us that the show is satirical in nature, this causes problems. They were planning on bringing him into the third season, but couldn't find the right voice actress.
Avatar The Last Airbender goes out of its way to have a balanced roster of male to female characters, but the action figures of the series, are only of the male characters (even minor or nameless male characters get action figures ahead of the lead females). They eventually announced a figure for Katara in the second line, but then the whole line was cancelled and they haven't made any since.
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... Half the cast of Cybertron, for example. How long did it take for Evac to stop doing anything? Four episodes?
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.
And speaking of the movie, several characters had this happen to them. Bluestreak, Sunstreaker, Huffer, Thundercracker, Skywarp, and Bombshell appear in cameos with no lines, Shockwave appears in two scenes and is implied to die, Shrapnel and Kickback are reduced to Butt Monkeys and then rebuilt, Mixmaster, Scavenger and Long Haul do not speak, Blitzwing gets one scene for himself, Ironhide, Wheeljack, Brawn, Prowl, Ratchet and Windcharger are killed off and Sludge, despite being a major character, has no lines. In fact, Spike and the three major Autobots who survive the movie (Bumblebee, Jazz, and Cliffjumper) have more minor roles. Perceptor and the four other Dinobots (Snarl mysteriously receiving this treatment) are the only pre-movie characters to do much of anything in the post-Autobot City scenes. Even Optimus Prime is only around for the first half of the movie. Plus, some characters don't appear in the movie at all, leaving their fates uncertain.
Sandy Cheeks, a major character, only has a cameo and one line in The Movie, and no importance to the plot, and rarely appears in the Paul Tibbit seasons, to the point where she is forgotten, unlike the original series. They even forgot her name in one episode (she was called "Sandy Squirrel" in a later episode).
Larry the Lobster got some decent roles early on, but seems to have been forgotten as a character in later seasons.
A lot of recurring characters from the show seem to have been abandoned most of the time due to the unhealthy amount of focus with Mr. Krabs and Plankton. Such as Sandy, Gary, Mrs. Puff, Pearl, Larry, The Flying Dutchman, Karen, and Patchy the Pirate.
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.) The movie that Craig didn't get to make supposedly would have re-dressed the balance. According to this Wikia article, the much-ignored Nadine would have finally gotten some screentime in it.
Lots of The Simpsons characters, especially those from the show's first two seasons. For instance, in Season 1, Lewis was regularly seen with Bart and Milhouse, and was explicitly said to be Bart's 2nd best friend. He was quickly demoted to background character, and even those appearances are getting rarer.
Bart: I guess this is the end, Wendell. Lewis: He's Wendell! (points to Wendell) I'm Lewis! Bart: Well, whatever. Just tell Wendell I said bye.
And another was hung in "Homer to the Max";
Homer: I don't understand what happened. Why did they change Homer Simpson's character from cool to stupid? Lisa: The first episode was just a pilot, Dad. Producers fiddle with shows all the time! They change characters, drop others, and push some into the background. (Mr. Largo and the Capital City Goofball walk by in the Simpsons' background window as Lisa says this.)
For similar reasons, Lunchlady Doris stopped appearing after the death of her VA, Doris Grau. This lasted about a decade, until someone noticed Tress MacNeille does a rather good impression. (Well, she does an impression, at any rate.)
Marge's mother has also largely vanished. This is a matter of the stress on Julie Kavner's voice; whenever Mrs. Bouvier appears, so do Selma and Patty, and Kavner's voice takes a beating.
Bill and Marty (Springfield's resident DJs) have also slowly disappeared (they stopped appearing after season 15's "Fraudcast News", only to return for season 18's "The Boys of Bummer", and then vanished again)
Guest characters voiced by celebrities sometimes stick around as mute extras. This is especially common for classmates of Bart and Lisa like Alex (voiced by Lisa Kudrow), Allison (voiced by Winona Ryder) and Jessica Lovejoy (voiced by Meryl Streep.)
Another boy named Richard could be seen with Lewis, Milhouse, and Bart in the earlier episodes, but was quickly shoved into the background as well.
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.
Terrance and Mac's mom had very few appearances in the fifth and sixth season of Fosters Home For Imaginary Friends. Goo had very little screen time in the later seasons too.
Connie (a.k.a. Kahn Jr.) from King of the Hill was a fairly important character early on and a love interest for Bobby, however they broke up in the sixth season, but she still had major roles from time to time, but starting with season 9, her role was greatly diminished and wasn't seen very often. In several later episodes where her parents were the main focus, she was nowhere to be seen, and when she did show up, her role was very minor— usually limited to a few lines of dialogue.
What made this really odd is that Lauren Tom voiced both Connie and her mother Minh, so it wouldn't have been much of a hassle to toss Connie into a scene with her parents.
One might forget that the Griffins on Family Guy even have a daughter, with as little screen-time as Meg gets these days. This is even parodied in Family Guy Presents: Laugh It Up, Fuzzball, where she's in a couple of seconds of the special, tops, and as a giant space worm.
Parodied in "Something Something Something Darkside":
Meg: How come I never have any lines in these things?
Peter: Shut up, Meg.
Played with in later seasons, which seem to give her more side stories and Deconstruct her Butt Monkey status.
One might forget too that the Griffins even have another son besides Stewie. Chris' screentime is even lower than Meg's.
Hayley from American Dad!, particularly from Season 3 on. Compare her screen time with Francine's, Steve's, or Roger's. Arguably because the show switched from politics driven to character/story emphasis, and her personality wasn't much developed other than as the strawman liberal.
Also, some say Klaus has had very little screentime or lines in later years, but he still appears about the same, and has about the same number of lines he always has had (albeit both have always been very brief). One could argue he was, and always has been an extra.
King Koopa of all characters got this treatment in the Super Mario World cartoon. After appearing in nearly every episode of the first twoaninmated 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.
Timmy Turner's best friends, Chester and A.J. In the beginning, both of the boys were major characters with early episodes that centered on Timmy's friendship with these two; they now appear very sporadically, often in non speaking roles. A.J. seems to have suffered the worst from this trope.
It's most evident with Veronica, Trixie's "best friend", after season five.
Vicky in general, despite being heavily featured in the opening theme, appears as the antagonist less often nowadays compared to the Anti Fairies, Dark Lazer, and especially Mr. Crocker.
Timmy's Love Interests have seemingly suffered a similar fate: Trixie hasn't made a single speaking appearance since her voice actor retired in 2009. Tootie also, besides the live-action movies, hasn't appeared in the series since Season Seven.
Every character in the X-Men animated series had this happen to them. With the obviousexception... Two notable X-Men, Colossus and Nightcrawler, were regulated to cameos and guest appearances. This was thanks to them having been a part of Excalibur in the comics at the time, as well as Marvel's desire to focus on the more current 90's members of the team. Poor Kitty Pryde didn't appear at all.
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.
Tony Toponi and Bridget get demoted to background characters with a couple blink-and-you-miss-it cameos in An American Tail: Fievel Goes West after having played a pretty large role in the first film.
In Total Drama Action, the second season of Total Drama, Ezekiel, Eva, Noah, Katie, Tyler, Cody and Sadie were reduced to this (though arguably Ezekiel was barely more than an extra to begin with.)
In World Tour, four of those characters (Ezekiel, Noah, Tyler, and Cody) came back, but Geoff, Trent, Justin, and Beth, the previous season's runner up, were demoted in their place. And then of course Eva, Katie, and Sadie were not saved from this status.
In Revenge of the Island, Noah, Katie, Sadie, Courtney, Trent, Harold, Leshawna, Cody, Sierra, Alejandro, Beth, Justin, Eva, and Geoff were the only members of the original cast who didn't get individual cameos appearances after the first episode.
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.
Bunnie and Rotor are also fairly minor characters in the second season, with Sally and Antoine played as Sonic's more prominent allies.
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 the original 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.
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.
In early episodes of Phineas and Ferb, Django was supposed to be one of the main members of the title characters' group of friends. He got one episode ("Oil on Candace") sort of centered around him, but by season two he's only shown up in a few background shots.
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.
While we're dealing with Winx Club, and related to the Avatar figures example above: Under Mattel, Tecna and Musa were never represented as much as their fellow Winx in the doll lines. Some lines even omitted Tecna altogether, but their biggest sin was when they released the Singsational line... and released Musa (the musical fairy) behind the others. (Also, the only doll they ever released outside of the main fairies was one of Sky.) Italian licensee Giochi Preziosi, and later Rainbow Toys, appear to have been a bit better with equal representation in the doll lines, at least with the fairies (they even made dolls of the Trix).
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 family, Matt and Mary McGinnis, while not as prominent as Dana, also had less and less screentime as the series continued.
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.
In The Cleveland Show, the daughter Roberta was demoted to this a little into the first season, having very little screen time and less dialog. She is also currently the only member of Cleveland's new family that hasn't met the Griffins. However this was mainly due to her voice actress on maternity leave.
In Blazing Dragons Season 2, Princess Flame plays a major role in some of the first episodes, but in later episodes she sometimes does not appear, and rarely speaks any lines when she does (appearing in a few episodes with no lines at all).
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 she's reduced to a background character who does little, says little, and has no effect on the 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.
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.
While the Urpneys still get a lot of Sympathetic P.O.V. in the pilot for The Dreamstone, Rufus is spotlighted as the obvious main protagonist and gets the most Character Development. The majority of later episodes place the Urpneys as Villain Protagonists with Rufus and the other heroes usually getting far less screentime and focus than them. Even their boss Zordrak is demoted, rarely appearing other than to chew out his minions for failure at the beginning and end of each episode (and sometimes not even that).
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 3-4 episodes of the second (to be fair, Hank had personal problems he had to deal with).
Rarity in season three of My Little Pony: Friendship is Magic. Introduced as one of the main characters, someone watching in season three would probably not know this, as she is the only main character without a focus episode (even SPIKE got two to himself). Some have argued that she was becoming Demoted to Extra since season two. According to Word Of God, Rarity's lack of a spotlight episode was the result of a retool of "Spike At Your Service", since the writers disliked her portrayal, changing the lead character and dynamic completely. A return to spotlight has already been confirmed for Season Four.
Cheerilee got hit with this hard. Seasons 1 and 2: Not a main character by any means but still a recurring character. Season 3: Background extra. Of course, when you only have half a season, someone's gotta go.
Thomas and Friends 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.