Terrytoons had three characters in their Deputy Dawg show that spun off into their own series. A little blue space alien became Astronut, a Russian mouse named Mischa became Macon Mouse, and the Long Island Duckling became Duckwood. Astronut was paired with a human named Oscar Mild, Duckwood was paired with a con-artist mule named Donkey Otie, and Macon Mouse was among the gang in Possible Possum.
By Series:
The credits of Season 2 in Abby Hatcher features the credit of someone named "Farmer Jo", who hasn't appeared yet several episodes in.
Adventure Time: The Lich first appears in a quick cameo in the episode "His Hero" in season 1, well before his official introduction as the Big Bad of season 4.
Ralph the Guard, a regular on Animaniacs, made his debut as an unnamed, surprisingly competent security guard in several episodes of Tiny Toon Adventures. Likewise, World's Oldest Woman and Lucky Bob, two characters on Histeria!, made cameo appearances in Animaniacs segments years before their own show premiered. Histeria's version of Joan of Arc (which was actually just a caricature of Shirley MacClaine) also first appeared on Tiny Toons.
Most of the cast of Animaniacs (excluding the Warners, Dr. Scratchinsniff, Hello Nurse, and Mr. Plotz, all of whom were formally introduced in the first segment) appeared in the segments "The Monkey Song" and "Nighty Night Toon" before getting their own segments.
Avatar: The Last Airbender: Princess Azula makes a brief appearance halfway through Season 1, in the episode "The Storm". She's seen in the audience during Zuko and Ozai's Agni Kai, in which the former receives his signature facial scar, and is notably the only one in the crowd smiling as Zuko screams in pain. She is also in the opening theme as the firebender, albeit cast in shadow as a silhouette. She isn't identified as Zuko's sister until the last scene in the season finale, and even then, we don't learn her name until the season two premiere.
The Batman's second episode "Traction" features a flashback to the night Bruce's parents died, with a young red-headed and mustachioed police officer informing Bruce that Alfred's there to pick him up. According to producer Jeff Matsuda, this was indeed supposed to be a young Jim Gordon, who would properly debut in the Season 2 finale, "Night and the City" with a side of Remember the New Guy? as dialogue by Alfred and Gordon himself suggested Batman was already in an alliance with the recently-appointed Commissioner despite only appearing in said flashback beforehand. Also, Matsuda denied that the flashback was done as a tie-in to Batman Begins, which started a trend (including being Ret Canoned in the comics, as well as the Batman: Arkham Series and Gotham) of having the future Batman and future GCPD Commissioner meet the night the Waynes' died.
Harvey Dent is established as Gotham's DA and a friend of Bruce Wayne in two episodes before he has the accident that melts half his face off.
Matt Hagen is mentioned on a magazine cover as the "Man of a Thousand Faces" a couple episodes before his debut episode and transformation into Clayface.
In Beware the Batman episode "Monsters", Special Crime Unit captain Dane Lisslow briefly appears as he directs his forces against Batman, Metamorpho, and the armored criminals. Who knew this unassuming, reasonable officer would end up becoming a close friend of Bruce Wayne a few episodes later? Or that he'd end up being Deathstroke, the Big Bad of the second half of the season?
Due to the show's abrupt disappearance from Cartoon Network, Man-Bat and Killer Croc appeared in the tie-in comics several months before their introductory episodes aired on television.
Wholesome Foods, the store run by the Greens' rival Chip Whistler, makes a cameo as the store the Greens get their steak from in "Steak Night" before its official debut in "Supermarket Scandal".
The Barry Cuda song was briefly heard in "Gridlocked" prior to its titular episode.
Bonkers's second partner Miranda Wright makes an extended non-speaking cameo as the Chief's uniformed secretary in "Hear No Bonkers, See No Bonkers", which, though animated later, is the fourth episode chronologically.
The MTV show The Brothers Grunt stemmed from a station ident where one of the characters craps out the MTV logo.
Mr. Bumpy, Molly Coddle, and Squishington, the three central characters of Bump in the Night, were featured in a series of stop-motion bumpers that aired on ABC in 1993 before they got their own show.
Carmen Sandiego: The poisoner from Season 2 makes a non-speaking appearance in Season 1's "The Vermeer Caper."
In the episode "Snapmobile", Craniac 3 makes a brief cameo before becoming the major villain in "Future Zone". Coincidentally, both episodes ended up in the same half-hour, though "Snapmobile" aired before "Future Zone".
Michelle Norwegiano makes a cameo in "The Wiggies", watching Penny talk about the titular creatures on a news report (Michelle herself having her hair eaten by them) before she's properly introduced in "The Skrawl" (however, Nickelodeon screwed up the episode order and aired the latter before the former). She also has a slightly different design (possibly an earlier model) and she's voiced by Hynden Walch instead of E.G. Daily.
Non-character example: Rudy was first shown in his season four outfit in a comic in the July 2004 issue of Nickelodeon Magazine, a year before season four began airing.
The Codename: Kids Next Door villain Potty Mouth debuted as a background character in the episode "Operation: M.O.V.I.E." before making his first full appearance in "Operation: A.W.A.R.D.S."
All of the monsters and antagonists seen in the opening of the first season of Courage the Cowardly Dog wouldn't appear within the series until the second season: Schwick from the episode "Courage in the Big Stinkin' City", the McPhearson Phantom from the episode of the same name, Robot Randy from the episode of the same name, the mummified Mayan baker from "Courage Meets the Mummy", Carmen from "Serpent of Evil River", and one of the banana people from "1000 Years of Courage".
The Dick Tracy Show's Go-Go Gomez made his first appearances in two Mr. Magoo shorts prior to the series: "Requiem for a Bull" and "Fuel in the Sun". He is referred to by his full name in both.
General Amaya can briefly be seen from behind at the end of the opening narration, but does not appear properly until episode 4. She's also mentioned by Harrow in the first episode, but without any indication that she's a member of his own family.
Aaravos isn't properly introduced until season 2, but he's the narrator of the prologue, briefly appearing in it alongside other elves, and it's also his hands unfurling the map in the show's intro sequence
Ethari, Runaan's husband, is shown in two post credit scenes in season one, though he doesn't show up until early in season three.
Family Guy: In "Holy Crap," Vern (one of the vaudeville guys) makes his first appearance after Peter bails out his father Francis. In Season 4, he (and Johnny, who wasn't introduced until then) would make several appearances.
In the Futurama episode "I Second That Emotion" Leela's parents can be seen in the background among the mutants in a few scenes, they would be properly introduced two seasons later.
Godzilla: The Series: Before Ts-eh-Go's appearance in "Where Is Thy Sting", a smaller but otherwise identical mutant scorpion named Scorpio briefly appears in the episode "Freak Show".
"The Time Traveler's Pig" and "Dreamscaperers" both give a cameo to the Author, Stan's twin brother Ford. He's the "Stan" who pokes his head of the Mystery Shack in the former episode; in the latter, he can be seen as a kid, reading in the bleachers during Stan's wrestling memory.
The time traveler Blendin Blandin from that same episode appears in the background of three prior ones, cleaning up the anomalies that Dipper and Mabel caused when they accidentally went to the past.
Bill Cipher flashes briefly at the very end of the theme song, and images of him (or triangles in general) can be seen all around the Mystery Shack. His first proper appearance is in "Dreamscaperers," the penultimate episode of season one.
The titular character of Hailey's On It! appeared in chibi form in a video celebrating the history of Disney Channel, several weeks before her show officially debuted.
The beginning of the Hazbin Hotel pilot features quick appearances by many characters who wouldn't receive real introductions until the series proper, including Lucifer, the Vees, Carmila, Zestial, Adam, and Lute.
Hilda: Frida and David make a small appearance in the second episode when Johanna takes Hilda on a tour of Trollberg, before both are fully introduced in the next episode.
House of Mouse for some reason portrayed Ronno as a fawn instead of an adult deer since Bambi is always portrayed here as a fawn as well. Young Ronno won't make his official appearance in any animated Disney work at all until the release of the Direct to Video film Bambi II.
In Episode 3, Multi-Paul, Dupli-Kate's twin brother in the comics, appears as a prisoner.
In Episode 8, Omni-Man's exposition over the history of the Viltrumite Empire features a brief cameo from Conquest near the end.
Jackie Chan Adventures: Well before Tarakudo officially appears in Season 4, his face is first shown in Season 2, in the form of his Mark of the Beast on the cover of the Demon Archive which turns Jade into the Queen of the Shadowkhan for an episode. The same mark is shown in Daolon Wong's cauldron when the dark chi wizard summons the Ninja Khan to serve him (it's the latter spell gone wrong that directly brings Tarakudo into the fold in the Season 4 premiere).
Pretty and Eugly appear in the theme song from the very first episode of season 2, but are only introduced in the second episode of this season.
Stumpy's seven younger sisters are gradually introduced throughout season 5, but all of them appear together in the theme song from the first episode of the season.
Cotton appears a few times in Season 1 through short flashbacks and dreams before his proper introduction in "Shins of the Father."
Lucky's debut was just as another minor member of the group of "rednecks" Kahn befriends in "The Redneck on Rainey Street." He becomes a regular starting in the next season.
Scheming politician Tarrlok appears in a photograph in a newspaper Korra uses for target practice in episode two, before getting his proper introduction in episode four.
Future Big Bad and Well-Intentioned Extremist Kuvira appears during a metalbending dance routine halfway during Book 3, after which she receives some minor speaking roles. She's not named until the season's penultimate episode, where her introduction coincided with a Scare Chord that instantly foreshadowed her future villain status in the show's final season.
In the Legion of Super Heroes cartoon, before he joined the Legion and coined his superhero name in The Substitutes, Matter-Eater Lad was a competitor in the Space Olympics episode "Champions." The other, rejected lame-power applicants also showed up in Lightning Storm for comic relief. Ultra Boy makes his first appearance as a competitor in Champions, as well.
Looney Tunes: In the short The Draft Horse, a soldier scrubs down the titular horse. Said soldier would later gain the name Private Snafu and star in a number of his own shorts, which were made specifically for the military.
The Loud House: Lincoln's female friend Stella made a brief nameless and faceless cameo in "White Hare" before being properly introduced in "Be Stella My Heart".
A few songs in Magic Adventures of Mumfie came out on CD and cassette before they appeared in the show, with "Home" and "Real Eel Electricity" being released the earliest. All songs from the show's CD after "Ocean of Sleep" are also this trope, as when the CD was released, the episode featuring the Pirate Song would air a week later.
Sort of inverse to the above example — part of Major Lazer's song "Powerful" played in the background of the Major Lazer episode "Vampire Weekend" nearly 8 months before the full track was officially released on an album.
In its second season, Marsupilami frequently appeared in The Disney Afternoon's commercial bumpers a year before Disney released his cartoons as part of the Raw Toonage series.
Magnus Hammersmith is seen in a flashback during the Season 3 premiere of Metalocalypse, long before he takes on a prominent role in Season 4. Also, Ishnifus Meaddle is quoted on the back of the Season 3 DVD. He doesn't appear until the fourth season finale.
Molly of Denali: In "Bye Bye Birdie," Tooey's mother Atsaq makes a cameo, but she is not properly introduced until later in the season.
Adventures in Ponyville showed Twilight as able to grant functional wings with magic, though they were a very different kind of wing (proper pegasus wings) than the ones she gave Rarity (butterfly wings made of gossamer and dew).
The Cutie Mark Crusaders — Apple Bloom, Scootaloo, and Sweetie Belle — appear huddled together for a short moment at the end of the first episode (Apple Bloom had already been introduced earlier, but the other two wouldn't officially meet her until episode 12).
Sweetie Belle also appears briefly at the beginning of Call of the Cutie due to an animation error, where an anonymous pony based on her model should've been shown.
The Mane Six's Rainbow Power forms from Twilight's Kingdom Part 2 were present in the toyline up to a full year beforehand (albeit without the enhanced hairdos).
At the very end of part 2 of "The Crystalling" as the Friendship Express is leaving the Crystal Empire, a small speck of what appears to be a Changeling can be seen in the distance to the far left; it is Thorax, who plays a larger role in the season's main arc much later.
Numberblocks: Eleven makes an appearance in "The Wrong Number", the episode prior to her introduction. She can be seen in the flashback that Nine had, walking down the sidewalk.
OK Ko Lets Be Heroes: Professor Venomous is initially seen on a POW card, several episodes before his proper introduction in "We're Captured".
When Eda is looking through her photo album in "Wing It Witches", a picture of her and Raine during their school years is clearly visible, (though the latter's face is covered by a ticket). Raine wouldn't be properly introduced until the following season in "Eda's Requiem".
"Agony of a Witch" has the Golden Guard appear in two scenes, as well as another in "Young Blood, Old Souls", before being properly introduced in "Separate Tides".
Also in "Young Blood, Old Souls", the flashback to Eda and Lilith's time as students at Hexside has Amity's parents in the background (after only being shown as silhouettes in "Understanding Willow"). Darius, the head of the Abomination Coven, can also be seen in the background before his introduction in "Eda's Requiem".
Speaking of Coven Heads, they all appear briefly in the opening scene of "Hunting Palismen". This includes Terra Snapdragon, the head of the Plant Coven, whose proper introduction is in "Follies at the Coven Day Parade".
Heffer from Rocko's Modern Life first appeared on an MTV bumper in 1989 (where the flew out of a TV with the MTV logo branded on his butt) before being a deuteragonist for that show four years later.
Double Trouble appeared in the background of the Princess Prom at season one, three seasons before their significant appearance.
Octavia, who plays a notable role in the episode "Boys' Night Out", can be seen among the Horde officers watching Shadow Weaver's dressing down in "The Beacon".
Rich Texan is frequently associated with later Simpsons episodes, but his first appearance in the series was the Season 5 episode "$pringfield (Or, How I Learned to Stop Worrying and Love Legalized Gambling)".
In the second season episode, “Dancin’ Homer”, the owner of the Springfield Isotopes is revealed to be Antoine “Tex” O’Hara, who dresses and sounds exactly like the Rich Texan (and is even voiced by the same actor), so it’s possible that this character was a prototype, perhaps inadvertently.
Skull Island (2023): Recurring character Sam first appears in the premiere among the group of mercenaries that Annie fights with during her escape. He doesn't have any lines in this episode, and we don't get his name or characterization until later on in the series.
Five years before the first episode aired, creators Trey Parker and Matt Stone made a short film called The Spirit of Christmas, which was made using construction paper and had prototypical and mostly unnamed versions of the show's main characters. Three years later they made another Spirit of Christmas, this time with the show's main characters properly named, characterised and recognizable, and also added Wendy Testaburger as an unnamed, non-speaking cameo.
Butters had been a background character since the very first episode. He spoke in Season 2's "Roger Ebert Should Lay Off the Fatty Foods" and "Clubhouses", but he lacks his usual stuttering Southern accent.
Like Butters, Token also first appeared in the first episode, but was never named or spoke until around the third season, after which he became a recurring character. In fact, he was depicted as white in some of his Season 1 appearances.
Timmy had a scene in "Tooth Fairy Tats 2000", and became such an immediate Ensemble Dark Horse that he quickly gained his own day in the limelight, "Timmy 2000".
In the HD remaster of "The Jeffersons", Scott Malkinson can be seen in line for cotton candy in the playroom. His original debut wasn't until "Elementary School Musical" four seasons later.
Not only does Eddie Brock get an extensive role (influenced by his role in Ultimate Spider-Man), but we also see Norman and Harry Osborn, The Sandman, The Rhino, The Shocker, Doctor Octopus, Curt Connors, and, possibly, Frederick Foswell, as supporting characters or minor villains before their inevitable darker turns, as well as every named schoolmate Peter had in the comics, and Gwen Stacy, whose fate is well known. Felicia Hardy also cameos in costume in a Halloween Episode before her official introduction.
Of specific note is Quentin Beck, who's introduced in Season 1 as Chameleon's henchman before adopting his Mysterio persona in Season 2. Naturally, when Spider-Man unmasks him, they make fun of it:
Mysterio: Don't pretend you've forgotten! I was the Chameleon's right-hand man!
Spider-Man: Ooooohhh... right... on the boat... you were... you were dressed as the crewman!
Mysterio: AS THE WAITER!!! You remember! 'Demitasse, Spider-Man?'
Spider-Man: Sure, sure, if you say so.
Cletus Kasady (Carnage's host) makes a cameo in the Christmas episode. Had the series continued he probably would've become a recurring villain.
Morris Bench (Hydro-Man in the comics) and Roderick Kingsley (Hobgoblin in the comics) also made appearances, and Mac Gargin (Scorpion in the comics) was mentioned at one point. Had the series continued, all three villains would have likely been used.
Miles Warren also makes recurring appearances as an amoral and cold-hearted scientist who gradually takes over Curt Connors' lab, during which time he makes it clear he's interested in gene-splicing by transforming Kraven the Hunter. Had the series gone on he would have become the supervillain The Jackal (who splices his DNA with a jackal late in the comics) and probably done their version of The Clone Saga. They also point out that he is the brother of Peter's high school science teacher.
Also, a girl seen at the school dance in the seventh episode later becomes a recurring character in the second season: Sha Shan, Flash Thompson's would-be-girlfriend.
The second episode of Spider-Man: The Animated Series featured Spencer Smythe, his son Alistar, Norman Osborn, and Eddie Brock, all working on a plan to capture and unmask Spider-Man, though only Osborn is aware they are doing as part of the Kingpin's larger plan to kill Spider-Man. While Alistar starts working directly for the Kingpin next episode, Eddie wouldn't become Venom until later on that season, and Osborn wouldn't adopt that mantle of the Green Goblin until Season 3.
Max Dillon appears in the eighth episode of Spider-Man: The New Animated Series as Peter's friend and classmate, and doesn't become Electro until the following episode.
Flats the Flounder had cameos in two Season 1 episodes of SpongeBob SquarePants, "Sandy's Rocket" and "Texas" before being featured in "The Bully". Possible Fridge Brilliance as "Sandy's Rocket" shows him as one of SpongeBob's alien hunting victims possibly giving him motivation for wanting to kick SpongeBob's butt in his featured episode.
Glossaryck from Star vs. the Forces of Evil makes two brief appearances in "Star Comes to Earth" and "Monster Arm" before making his proper debut in "Mewberty".
Star Wars: The Clone Wars: From the Pilot Movie, in the scene before Anakin and company land on Teth, a bald clone pilot with a black and red painted helmet is visible listening to Ahsoka brag about saving Anakin on Christophsis. We'll be introduced to this pilot, named Matchstick, in the first season episode "Shadow Of Malevolence".
The characters Ruby and Sapphire appear briefly in silhouette during "Fusion Cuisine" early in season one, when Alexandrite unfuses, twenty episodes before they'd receive proper introductions in "Jail Break".
Bismuth. "Lion 3: Straight to Video" in the middle of the first season has her gemstone seen floating among the many miscellaneous objects in Lion's mane. Then, early Season 3 episode "Same Old World" has a brief scene of Bismuth poofing Lapis Lazuli during a flashback. We finally meet the character near the end of the third season.
The title sequence for Future shows several antagonists that appear throughout the series. They are Jasper, two Lapis Lazulis, a fusion of Aquamarine and Eyeball Ruby, a cactus that Steven brought to life, White Diamond being controlled, and a large monster who turns out to be Steven himself.
Storm Hawks: Harrier and Suzy Lu (with their respective squadrons visible farther in the background) and the Screaming Queens and Buff Buzzards all make cameos in "Age of Heroes" amid the combined assault on Cyclonia. They respectively play relevant roles in later episodes.
The first episode of Stretch Armstrong and the Flex Fighters features Officers Reynolds and Santos as two members of Rook Unlimited's security: The former becomes an unexpected major ally to the Flex Fighters near the end of the second season, while the latter becomes a villainous cyborg obsessed with revenge at the beginning of season two. In addition, when the Flex Fighters defend the train from Multi-Farious, you can see Rook's mysterious Flexarium test subject in his disguise as an unassuming civilian.
Eugene's comb, the first thing he ever stole, appears in "Eugene vs. Cassandra" before being properly introduced as a plot point in "Big Brothers of Corona."
The series' true Big Bad Zhan Tiri makes a brief first appearance in the form of the blizzard that nearly destroyed Corona when Rapunzel was put in charge in "Queen For a Day".
Teen Titans (2003): Silkie can be briefly spotted in a few season 3 episodes before being properly introduced in “Can I Keep Him?”, serving as foreshadowing that Beast Boy kept the moth larva he held in season 2’s “Date with Destiny”.
Laserbeak from Transformers: Animated actually first appeared in the form of a toy that is packaged in the same blister pack as Animated Soundwave. Laserbeak himself didn't appear in the cartoon until season 3. (This was due to the show's creators not being informed that Soundwave came with a Laserbeak toy.)
In Ultimate Spider-Man, Curt Connors has a recurring role in the first season as a SHIELD scientist. He doesn't become the Lizard until the second season.
The Venture Bros. uses this form of the trope regularly. Many easily-missed background characters gain major plot relevance down the line. Sometimes it's only a few episodes later (such as the Investors appearing in the background of an episode before their official introduction,) while other times it's many seasons later (such as August St. Cloud, who was a frequent background character dating all the way back to season 1, but wasn't officially introduced until season 5.)
The Prince of Brakmar first appears in "Kriss Krass", before playing a small but important role later on in the season.
In "The Council of Twelve", the woman next to the Osamodas King is actually princess Aurora, who's eventually married to Armand in Season 3.
The Yogman Twins Lenny and Denny appear in the pilot episode of Whatever Happened to... Robot Jones? as background characters before being properly introduced in the episode "Electric Boogaloo".
The cartoon had background appearances from Mystique, Pyro, Avalanche, Blob, Omega Red, and Captain America at least an episode (or in Cap's case, a couple of seasons) before they actually played some sort of role.
The tie-in comic series to Young Justice (2010) came out alongside the show and is officially canon to it. As a result, it premiered several characters long before they made their debut on the show such as the Terror Twins, The Joker, Kaldur'ahm's parents (Sha'lain'a and Calvin Durham), and Wyynde. The latter three really stand out because they appeared on the show seven years after their comic appearances due to the show's cancellation and revival.
for the first season of The Disney Afternoon, about a year before their own show started airing.
