Adventures of the Gummi Bears: Igthorn and Lady Bane team-up in two episodes to plan how to get rid of the Gummi Bears. Igthorn's crush on Lady Bane is a running gag, especially because it's not mutual.
Unlike other villain team ups however, these villains actually trusted each other (although some of them didn't get along very well). The only reason Tee betrayed them was because they all belittled him, and Zix and Travoltron only left because Tee convinced them to do so. Other than that, the only reason they lost was because Jimmy outsmarted them all one at a time.
Aladdin: The Series had a villain team up episode where Abis Mal had gotten some magical device but didn't know how to get it where he wanted it, and Mechanicles just happened to walk into the Bad Guy Bar with some mechanical insects at the right time.
As well as another one where Amin Damoolah is hired by Mozenrath to turn the Sultan into a gold statuette and smuggle him out of the palace. It works pretty well until Amin panics and turns into a griffin, the one creature who can change the Sultan back.
This trope was spoofed MERCILESSLY in the Aqua Teen Hunger Force episode "The Last (Expletive Deleted) One of 2003" when the ATHF's "Rogue's Gallery" formed a group called Monday Tuesday Wednesday Thursday Friday Saturday Sunday.
In Aquaman, Torpedo Man, the Claw, and Magneto teamed up against the Aquaman family, and one episode had Black Manta, the Brain, and Vassa team up as well.
Atomic Puppet: The Season 1 finale "The Big Shift" sees one between Atomic Puppet's two most prominent foes, Mookie and Professor Tite-Gripp, in which they lead an enormous prison breakout to defeat Atomic Puppet. Several lesser villains also appear in the breakout, but have no prominent roles in the episode.
Harley Quinn and Poison Ivy first became partners in the "Harley and Ivy" episode. The two teamed up frequently in this and various other Batman-related media, including the mainstream comic book, usually when Harley was separated from the Joker (either because he's in Arkham or after a fight with him).
Also in the episode "The Strange Secret of Bruce Wayne": The Joker, The Penguin and Two Face team up in order to buy the secret of Batman's real identity from Dr. Hugo Strange. And when Batman secretly tricks them into thinking Strange was playing them for the fool, they proceed to team up to take out Strange's goons and take Strange hostage to...teach him a lesson.
The episode "Almost Got 'Im" deserves a mention; The Joker, The Penguin, Two-Face, Poison Ivy and Killer Croc do not team up, but they play cards while telling stories of how they "almost got Batman".
The Trial is probably the episode of B:TAS with the largest number of villains teaming up; The Joker, Two-Face, Poison Ivy, Harley Quinn, The Scarecrow, the Mad Hatter, The Riddle, Killer Croc and The Ventriloquist, team up after taking control of Arkham to trial Batman.
The Joker and the Weeper in The Vile and the Villainous! Notable because Joker was actually proud of the Weeper betraying him to be the sole king of crime, only to be disappointed when Batman thwarts them anyway.
"Evil Under the Sea!" had Orm and Black Manta team up to take Atlantis away from AQUAMAN.
In "Back With a Vengeance," the second season finale of Ben 10, Ben's two worst enemies, Kevin 11 and Vilgax, team up. They play this trope as described, getting into two knock-down brawls with each other before ganging up on Ben. In the end, Kevin tries to steal the Clingy MacGuffin that Vilgax wanted off of Ben and leave Ben and Vilgax Trapped in Another World, instead of just killing Ben like he'd wanted to earlier. Then he goes right back to trying to kill Ben and, predictably, this is his downfall.
The next to last episode of the series had a group called the Negative 10 composed of most of the lesser villains Ben, Gwen, and Max faced through the series headed by new villain, the Forever King. They actually manage to work together rather well but end up outsmarted in the end.
Also, in the Alien Force season 3 finale, Vilgax teams up with Albedo in order to steal Ben's Omnitrix, so Albedo can go back to his original form, while Vilgax claims that he isn't interested in the Omnitrix anymore, and just wants to see Ben dead. Predictably, when Vilgax got the Omnitrix, he immediately betrayed Albedo.
In Omniverse, Vilgax teams up with Eon and Albedo to gather an army of evil alternate Bens to wipe all the good Bens from existence. Unsurprisingly, the device Vilgax uses for this targets every Ben except No-Watch Ben, so Eon and Albedo, being sort-of alternate Bens themselves, end up betrayed and erased. They get better thanks to No-Watch Ben, but even that didn't stop Albedo from teaming up with Vilgax to conquer Galvan Prime in a later episode.
Various villains would team up all the time in Captain Planet. Dr. Blight in particular often worked with one or two others on a project. The only time all seven of the main Eco-villains got together was in the two-part "Summit to Save Earth," with Zarm as the de facto boss.
Happens twice in ChalkZone. First, in "Double Trouble" when Skrawl and Craniac 4 team up to destroy the Chalk Mine and make Rudy their slave. Then in "Snap VS Boorat", Vinnie and Terry join forces to try to steal the magic chalk and expose ChalkZone.
The villains from Codename: Kids Next Door have done this quite a few times, justified since the heroes they're fighting are part of a military organization.
Cool McCool: "College of Crooks" featured the show's six villains—The Owl, Hurricane Harry, the Rattler, Jack-in-the-Box, Dr. Madcap and Greta "Green Lips" Ghoul"—joining at a disused college campus to conspire against McCool.
In the episode of Courage the Cowardly Dog, "Ball of Revenge", Eustace decides to take out Courage by gathering a group of villains from previous episodes in an alliance against that stupid dog (ignoring the fact that these villains have also endangered his own life in the past). The team up consisted of Katz, Le Quack, the Queen of the Black Puddle, the Cajun Fox, the Clutching Foot, and the Weremole.
Darkwing Duck had a tribute to/parody of the Sinister Six: the Fearsome Five, consisting of Megavolt, Quackerjack, The Liquidator and Bushroot, as led by Negaduck. In one Fearsome Five episode, Negaduck betrayed the other four and stole their powers for himself. The depowered fearsome four teamed with Darkwing to stop him.
In the revival comic, Negaduck has teamed up with Magica DeSpell.
Alvin the Treacherous and Mildew team-up at the end of season one and most of season two, due to the latter hatred of dragons and grudge against Hiccup and Stoic.
Alvin the Treacherous and Dagur the Deranged team-up too against Berk for a while but it doesn't take too long before Dagur turns against Alvin and took control of the Outcasts tribe.
Dragons: The Nine Realms: The fifth episode of Season 7 sees Big Bad Buzzsaw teaming up with recurring antagonist Wilma Sledkin allowing her to collect as much Dragoncite as she wants in return for translating the Book of Dragons. Their partnership seems to be permanent as it has not ended by the end of the season. During the season finale, Buzzsaw also added the Sky Torcher, the Big Bad of Season 3, to the mix, be it not as an equal but rather his minion.
In DuckTales (1987), the Beagle Boys were sometimes hired by Magica DeSpell or Flintheart Glomgold to help pull off their latest evil deed to be done to Scrooge McDuck. Eventually, all three parties teamed up to really rain on Scrooge's parade in a seven-issue story arc, Scrooge's Quest, that ran in Disney's DuckTales comic book (this was done seven years prior to the above-mentioned Don Rosa story). It's a minor case of What Could Have Been, though, as the Beagle Boys are just depicted as hired thugs for the other two, not with their traditional knack for over-the-top heists; and Magica and Glomgold don't really cooperate, they just agree to time their attacks on Scrooge at the same time. No real planning or joining of forces.
In DuckTales (2017)'s episode "Glomtales" Flintheart Glomgold realizes that the reason why Scrooge always win is because he has a family helping him, thus he recruits the Beagle Boys, Mark Beaks, Magica De Spell and Don Karnage to be his "family". They were pretty close to win and in fact they did win, once Louie joins them and the scheme he designs works, winning the bet. However Glomgold betrays the rest just to find out that due to not using his legal name all the money won in the bet goes to Louie.
Dynomutt, Dog Wonder: Six villains previously defeated by Blue Falcon teamed up to get rid of him. When they thought they got rid of the heroes, they divided themselves in three pairs to commit crimes and the heroes decided to trick the villains into meeting to capture all of them. (Blue Falcon believed that if he went and captured one of the pairs, the others would learn he escaped and would vanish.)
Evil Con Carne: Hector decided to found the League of Destruction. It started crumbling down when the villains argued among themselves over who'd lead them. By the time the hero assigned to destroy the league arrived at their headquarters, the villains were already beaten.
The Fantastic Four (1967) featured a team-up of the villains Klaw, Blastaar, and Molecule Man in the episode "The Terrible Tribunal".
The Flintstone Kids: Captain Caveman's enemies united themselves against him. They spent a whole day trying to destroy the hero, failing to even get his attention.
Xanatos and Demona work together during the five-part origin episode "Awakening".
In "High Noon", Demona and Macbeth try to free the evil personality of Iago/Coldsteel with the Weird Sisters manipulating them behind the scenes.
"The Reckoning" featured a surprise collaboration by David Xanatos, Demona, Fang, Tailog and the clones tossed in for good measure.
And the three part episode "Avalon" has Macbeth and Demona working for the Archimage and the Weird Sisters, again, to retake Avalon. In all cases Demona and Macbeth (sworn enemies) work together only because the Weird Sisters were controlling them magically.
Generator Rex: The episode "Enemies Mine" features a team-up between Rex's greatest enemies (except Van Kleiss), Hunter Cain, No-Face, and Gatlocke. Valve is there too, but Rex justifiably doesn't take him too seriously.
"Hercules and the Arabian Night", a crossover episode of Hercules: The Animated Series with Aladdin: The Series had Hades team up with a recently Jafar in an attempt to kill one another's arch nemesis by pitting them against one another. While it was a deadly alliance (the heroes almost killed each other), the heroes figure out they were being played and Hades drastically underestimated Aladdin's wit, resulting in Jaffar's staff being broken and him being dragged into the Styx, becoming just another mindless soul swirling in eternity.
Johnny Test has the "Johnny Stopping Evil Force 5," a team-up of Wacko, the Beekeeper, Brain Freezer, Mr. Mittens (and his butler Albert), and Zizrar the king of mole-people.
Played straight, subverted, and lampshaded in Justice League. The original villain group that went up against the Justice League (the Injustice Gang), organized by Lex Luthor, was held together solely by money. This made it easy for Batman to play them against one another. The follow up team dropped the Injustice Gang moniker for the name "Secret Society". While the only thing really holding them together was their hatred for the League, their leader (Gorilla Grodd) was smart enough to take them through various team building exercises that kept them from falling apart, knowing full well that it's "not the easiest thing for loners, sociopaths, and psychos". The subversion is that he also uses his mental powers to sow seeds of distrust among the League: they fall apart in the same way a villain team usually falls apart, only to reassemble in the final act.
Justice League Unlimited plays it straight, but justifies it more than most villain teams. While the team does eventually break up in a brutal villain civil war, they're not joined by a hatred of the JLU. Rather, the villain team is a protection racket: since it's impossible to commit a crime against a group as efficient as the JLU, Grodd set up a co-op. All it cost was 25% of your gross criminal profits, and you could expect back-up if the League got in the way of your average bank robbery.
Earlier, they had a team-up of old Superman villains in the DC Animated Universe version of the Superman Revenge Squad. Unusually, this Squad was taken out not by Superman himself, but by his fellow Leaguers. Lobo even temporarily joined the League, and defeated Kalibak by burying him under an ever-growing pile of smashed cars until Kalibak finally said "Uncle".
Drakken and Lucre in "The Mentor of Our Discontent", Shego and Junior in "Two to Tutor" and "The Big Job", Shego and Motor Ed in "Car Alarm", and Drakken, Shego, and Hamsterviel in the Lilo & Stitch: The Series episode "Rufus".
The page quote comes from the episode "Steal Wheels", where Dr. Drakken and Motor Ed team-up. In this particular case the answer turns out to be 'because they're cousins' and also happen to share the same voice actor, John DiMaggio.
In the premiere of Season 9, Grogar forms a team up with Tirek, Crystalis, Cozy Glow, and Sombra. Sombra quickly defects, thinking he can take over Equestria by himself, and promptly dies again, which Grogar uses as an example to his minions as to why they need to team up to defeat the Mane Six. In the end, however, Grogar is revealed to be Discord in disguise, leaving the remaining three villains to be the last foes.
Abraham Kane and the Duke of Detroit in the two-part finale of Motorcity. Earlier, Abraham Kane and "Red".
My Life as a Teenage Robot: The episode "Legion of Evil" sees a few one-shot villains unite to defeat Jenny — The Hammer Brothers, the Mudslinger, Lancer, and Vladimir the lab rat.
Episode The Terrible 5 + 1 from Plastic Man has Solex breaking out of prison The Weed, Half-Ape, Clam, Computerhead and Disco Mummy to have them as his servants. When they get tired of him they join Plastic Man against him.
The Patrick Star Show: "Best Served Cold" surrounds five of the Star family's enemies teaming up. The ice cream man hates Patrick for his relentless pursuit for his Trademark Favorite Food, Squidward hates that Cecil never pays for his newspaper deliveries, Death feels that GrandPat has evaded him for way too long, Fentin is jealous of Squidina's success, and Gladys... doesn't actually have a grudge, but a sea fly does due to her always swatting flies. The team-up ends with all of them heavily injured, with Fentin pointing out they should have cooperated rather than just taking turns.
In Pirates of Dark Water the evil pirate captain Bloth teams up with Morpho the server of The Dark Dweller to kill Ren and his friends, but other than that, they have very different goals as Bloth wants to be rich and Morpho wants the End of the World.
One episode had to fight a team-up of Mojo Jojo, Him, Princess Morbucks, and Fuzzy Lumpkins that ultimately broke up because, in the end, they were just a giant parody of The Beatles ("The Beat-Alls"). This is how the Girls actually beat them-the villains actually work very well together and the Girls can't beat them in combat. Instead, they find a female monkey who causes the team to fall apart on its own after Mojo Jojo falls in love with her.
Another episode occurs where the Gangreen Gang breaks into the Mayor's office and uses the Powerpuff Hotline to trick the Girls into breaking into Mojo, Fuzzy and HIM's homes while they were just taking a nap, taking a bath, and working out, respectively. Needless to say, the three villains did not take it so well and, upon finding out the truth, teamed up to give the Gangreen Gang a brutal beatdown.
The Real Ghostbusters episode Xmas Marks the Spot, after Egon enters the Containment Unit to retrieve the ghosts of Christmas he faces all the previously trapped ghosts including Samhain, The Sandman, Killerwatt, Ghash, Wat, the Winged Puma and the Sleeping Ghost.
Sheep in the Big City features a team-up between General Specific (who wants to capture Sheep so he can be used in a sheep-powered ray gun) and Lady Richington (a sheep-hating woman who beats Sheep with her wig whenever she sees him) in the episode "The Wool of a People". After a law is enforced making it so that they can't hurt Sheep, the two work together to ensure that General Specific becomes mayor, with Lady Richington serving as Specific's campaign manager. Their intent was to repeal the pro-sheep law once Specific won the election so that they'd be able to harm Sheep again.
The Smurfs (1981), Gargamel teams-up with another villain in at least one episode for season including Bigmouth, Balthazar, Hogatha and Nemesis, generally with unfortunate consequences... for him.
The first time was when Rhino and Shocker teamed up against Spidey. It'd be the hero's end if Venom didn't consider himself The Only One Allowed to Defeat You.
The Insidious Six (first with Rhino, Shocker, the Scorpion, Doctor Octopus, the Chameleon and Mysterio; later, the Vulture would replace Mysterio)
The Scorpion and the Vulture
The Spectacular Spider-Man has Doctor Octopus form the Sinister Six with the Vulture, Electro, the Rhino, the Shocker, and the Sandman in "Group Therapy". They are well-planned, have removed all their old weaknesses, and their members go out of their way to avoid intentionally hurting each other. It nearly works, until Spidey's Evil Costume Switch kicks in and turns their abilities against each other, like using Shocker's sound gauntlets to incapacitate Sandman.
The second incarnation replaced Shocker with Mysterio and Kraven, while Doc Ock worked behind the scenes. This time the plan was to make Spider-Man Run the Gauntlet, facing two members at a time until he was too exhausted to fight. However, by now Spidey was experienced enough to barely defeat them all, though most of them managed to escape while he was distracted fighting the others.
Spider-Man (1967) shows why ganging up on the hero doesn't necessarily work any better than making him or her Run the Gauntlet in "To Catch a Spider". When Dr. Noah Boddy, an invisible man created by the show's writers, busts Electro, the Green Goblin, and the Vulture out of prison to get revenge on Spider-Man, they actually take the time to weaken Spider-Man beforehand by knocking out his spider-senses and reducing his Super-Strength. Unfortunately, the villains are also shown constantly arguing with one another. Spider-Man exploits this by using ventriloquism to make it sound as if they're insulting each other, and the villains end up taking each other out.
Stretch Armstrong and the Flex Fighters: In the interactive special "The Breakout", some of the viewer's decisions can lead to a villain team-up as the final battle. The team-up consists of either Quick Charge and Circuit Stream, or Quick Charge and Multi-Farious, if the viewer avoided facing or taking down these villains initially.
In "Knight Time", with Bruce Wayne and Batman missing, Bane, the Riddler and Mad Hatter were planning to do this before Superman, impersonating Batman, and Robin (Tim Drake) stopped them.
The Superman/Aquaman Hour of Adventure episode The Trio Of Terror: Black Manta, Queen Vassa, and The Brain join forces... but not for long.
Happens a couple of times on Sushi Pack. In "The Yam Yakkers," Titanium Chef, Oleander, and Sir Darkly team up to create the titular creatures, and in "Fair Share For Sure," Apex partners with Sir Darkly to fund his Fountain of Youth-style laser.
In "Night of the Dark Kat" Dark Kat teams up with super-hacker Hard Drive to steal the Turbokat and use it to hold the city hostage and ruin the SWAT Kats' reputation.
In the season 1 finale "Katastrophe" Dark Kat teamed up with Dr. Viper and the Metallikats to destroy the SWAT Kats. This plays the trope extremely straight because Dark Kat does betray his comrades as soon as they've (apparently) captured all the heroes, stalling them long enough to salvage the situation.
In A Bright and Shiny Future Pastmaster aligns with the Metallikats, giving them control of Megakat City in a Bad Future so long as they'll kill the SWAT Kats from the present. Betrayal plays in here, too, where the Metallikats have the Pastmaster disarmed as soon as he delivers the present SWAT Kats to the future, prompting Pastmaster to help the heroes stop them.
Teenage Mutant Ninja Turtles (1987) has the Rat King and Leatherhead teamed up to fight against the turtles in one episode. In "Night of the Rogues", Shredder hires the past villains (including the mentioned two) to eliminate the turtles.
The Rat King had a short team up with Don Tortelli in the episode "The Great Bouldini".
The second-string Transformers: Animated villains Professor Princess, Angry Archer, Nanosec, and first-timer Slo-Mo team up to form the Society of Ultimate Villainy (SUV) in the episode of same name. Said villains later teamed up with the Decepticon Swindle, furthering this trope.
The bounty hunter Lockdown regularly uses this trope, as he's a Dangerous Deserter who will partner with either Autobots or Decepticons to get what he wants (though it's usually the Decepticons who hire him).
Transformers: Prime has had a couple of team ups involving MECH. First, in "Crisscross", they allied with Airachnid in order to capture Arcee and Jack (Airachnid wanted revenge, MECH wanted the technology in Arcee's body). This ended when Agent Fowler showed up with reinforcements, and the villains fled separately. Then, in the "Operation Bumblebee" two-parter, they formed an alliance with a renegade Starscream, who offered his knowledge in exchange for a share of the energon they'd need for their own plans. This lasted until Silas got sick of Starscream, stole his T-Cog, and then ditched him. Then, in "The Human Factor", Silas, having been grafted into Breakdown's corpse after he was traumatically injured, decides that MECH has outlived its usefulness and destroys his own followers. Then, he attempts to join the Decepticons; but when his plan to get them a Kill Sat fails, Megatron decides to let Knock Out vivisect him.
The final season of Teen Titans builds to a climax as the Brain leads a team up of nearly every villain in the show's run to capture the Titans and all their allies. Except the relatively harmlessones.
Underdog did this in its final story, with Simon Bar Sinister teaming up with Riff Raff, Electric Eel, and Batty Man. In that event, Simon lead them to steal all the money in the world with his latest invention, the Vacuum Gun. Unlike most examples, they didn't betray each other, and were instead defeated by Underdog one by one in the final part.
The Venture Bros. had Baron Underbheit consider an alliance with the Monarch in order to defeat Dr. Venture, only to be stymied by union bylaws... which they laugh at and plan on teaming up anyway.
During the latter part of season 2 of Wakfu, Quilby enters the Shukrute and propose an alliance with Rushu, offering him the long-awaited chance to invade the World of Twelves. The unforseen presence of an entire fleet of steampunk submarines on the island where the portal opened lead to Rushu ending the alliance, believing that he was lured into a trap, but by that point both villains had what they wanted.
Although not originally, this happens on occasion on Wordgirl. Tobey and Dr. Two-Brains team up in Mousezilla, but their tandem quickly falls apart due to them mocking each other about Two-Brain's fear of cats and Tobey's love of Wordgirl. The Whammer has teamed up with Chuck (both reluctantly on the latter's part) in Thorn in the Sidekick and Escape Wham. Two-Brains and the Butcher team up in "The Fill-In" when one of Two-Brains's henchmen goes on vacation for a while, though the Butcher decides to leave because he feels that Two-Brains is picking on him. And in the recent episode Too Loud Crew, the Whammer & the Butcher joined forces thanks to a supervillain manual.
"Beyond Good and Evil", the four-part premiere of the second-to-last season of X-Men: The Animated Series has Magneto, Mr. Sinister, Mystique and Apocalypse teaming up together. However, Apocalypse initially lied to Magneto that his goal was to create a mutant utopia while his real The End of the World as We Know It plans clash with Magneto's, thus causing him and Mystique to turn against Apocalypse. Mr. Sinister is all game with "let's destroy everything!" though.
