When Prince Achmed from The Adventures of Prince Achmed stumbles into the volcano home of a powerful witch, she tries to strangle him. When Achmed finally manages to tell her that he was marooned there by the evil wizard, she says "He is my greatest enemy! And we are friends!" She then helps Achmed defeat the wizard.
In the Futurama movie "Bender's Big Score", not only is the Robot Devil providing music at Leela's abortive wedding, but they later team up with the robot mafia and evil robotic Santa to take Earth back from the scammer aliens.
Though slaying the dragon was never part of the bargain; he just had to recover the princess (normally he'd have to slay the dragon, but this time he just outwits her).
Also, she comes to help because Donkey, who she loves, asks her to.
In the first Toy Story, Woody and Buzz are initially bitter enemies due to Woody's jealously over Andy's affection, but when the two toys end up separated from the rest of the group, they must work together in order to get back home. They then become good friends for the rest of the trilogy.
In The Boss Baby, Tim agrees to help Boss Baby "just to get rid of [him]". Neither really likes each other, at least at first. Boss Baby agrees because he wants to leave just as much as Tim wants him gone so that he can get promoted.
The animated Hobbit exaggerates it for laughs. Thorin and the elf king go from charging each other spitting curses to "your people are as brothers unto mine" in less than a minute when the goblin army shows up. Only Bilbo thinks there's anything odd about this.
In The SpongeBob Movie: Sponge Out of Water, SpongeBob and Plankton team up to retrieve the Krabby Patty formula after its theft by someone other than the latter turns Bikini Bottom into a post-apocalyptic wasteland.