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.
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.
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 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 to Dr. Hugo Strange.
Famous episode Almost Got 'Im deserves mention, the villains do not team-up, just play cards while telling stories of how "almost got Batman", but is still considered a favorite of the series.
The Trial is probably the episode of B:TAS with the largest number of villains team-up, practically the entire Rogues Gallery except for The Penguin, Clayface and Ra's al Ghul, 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.
Double-subverted and lampshaded example in Evil under the Sea! Orm and Black Manta have teamed up to take atlantis away from AQUAMAN, and are outrageously gloating over the latter's capture...
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. Predicably, when Vilgax got the Omnitrix, he immediatly betrayed Albedo.
Actually various villains would team up all the time; 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 villians from Codename: Kids Next Door have done this quite afew times, justified since the heroes they're fighting are part of a military organization.
In an episode of Courage the Cowardly Dog, 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).
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 DuckTales, 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.
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 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.
The Gargoyles episode "The Reckoning" featured a collaboration by just about every major villain this side of David Xanatos, with a few clones tossed in for good measure.
An earlier example in the series is "High Noon", with a team-up between Demona, Macbeth and Iago/Coldsteel, with the Weird Sisters manipulating them behind the scenes.
A crossover episode of the Hercules television series had Hades team up with a recently deceased 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), organised 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.
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 partciular case the answer turns out to be 'because they're cousins'.
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.
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.
The Powerpuff Girls 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 becomes a literal Yoko Oh No and causes the team to fall apart on its own after Mojo Jojo falls in love with her.
Somewhat Hilarious in Hindsight as Paul McCartney has since come out and said that Yoko Ono was not the cause of the Beatles' break-up.
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.
The Smurfs, 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 1960s Spider-Man cartoon shows why ganging up on the hero doesn't necessarily work any better than making him or her Run the Gauntlet. 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.
Superman: The Animated Series: 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.
The 1987 Teenage Mutant Ninja Turtles 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.
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 harmlessones.
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.
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. 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 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 End Of The World 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.