In "Underworld Connections" of The Legend of Zelda episodes, when Link hangs upside down, his nightshirt falls over his head to reveal heart-printed boxer shorts.
These show up at least twice in Jackie Chan Adventures, first in a dream sequence, then when Chan loses his pants due to a bad luck curse and not wearing a belt in a Season 2 St. Patrick's Day episode.
Two examples from TaleSpin: A security guard stripped of his uniform (Baloo and friends swiped it for a disguise) in the "Baloo Thunder" episode, and an undressed partygoer in the "My Fair Baloo" episode, which involved the entire cast disrobing to construct a hot-air balloon from their clothing to fly them out of the jungle in which their plane crashed.
In the classic Teenage Mutant Ninja Turtles cartoon episode "Attack of Big Macc", the title robot causes a security guard's pants to fall down and reveal these boxers. As they're retreating, Raphael hangs a lampshade on this by responding to Macc's statement that he detests violence by saying "I kinda feel the same way about underwear with hearts on 'em!"
They're pink hearts, not red, but they appear on a cop and off of Gandhi in Clone High.
In the Kim Possible series finale, "Graduation," when Shego steals Dr. Possible's spacesuit to accompany Ron on the space shuttle to rescue Kim and Drakken, she leaves him in a pair of black boxers with pink hearts. He is not amused:
Dr. Possible: (banging on shuttle portal, yelling)
In the "Steerminator" episode of Darkwing Duck, Steelbeak the villanous rooster had his clothes burned off by recently Back from the DeadBig Bad Taurus Bulba to reveal a pair of boxers with chicken drumsticks on them.
A Running Gag on The Critic involved Jay Sherman losing his pants to reveal a pair of underwear with a (usually embarrassing) movie title printed on the rear (such as Back Draft and My Own Private Idaho). This is a subset of a Running Gag established in the pilot: he never buys clothes because studios always send him free items as payment for his movie reviews.
A Running Gag on KaBlam! was the introduction of the "surprising shorts," animated shorts that were not part of any of the show's recurring series. Whenever Henry announced the shorts, June would promptly run over and pull down his pants, revealing embarrassing boxer shorts before announcing "Surprising!"
In Danny Phantom, being an unfortunate teen whose powers are triggered by his emotions, his pants phase down, revealing his polka-dotted red boxers.
At another time, Danny's English teacher, Mr. Lancer, was revealed to be wearing pink boxers with little Shakespeare heads on them twice.
Blanka/Charlie in the USA Street Fighter cartoon is shown wearing lightning-bolt underpants after returning to human form, likely a visual pun on his electricity attack in the original games.
All over the place in Adventures of Sonic the Hedgehog: in fact, Robotnik's robot wife tells him, after breaking up with him, "And I'm gonna tell everybody you wear polka-dotted underwear!"
Tails even lampshades this when he questions the mayor's taste in boxers.
Belle's father Maurice accidentally pulls down his pants and is wearing pink polka-dotted underwear when we first see him in Disney'sBeauty and the Beast.
Ron Stoppable regularly loses his pants (and sometimes shirt) as a Running Gag for the series and his boxers are almost always either polka dots over a solid color or striped.
Drakken and Shego find a load of Dementor's stuff in the timeshare lair, including boxers with a skull motif.
In Phineas and Ferb one fight sequence between the nefarious Dr. Doofenshmirtz and his nemesis Perry the Platypus resulted in the loss of his pants, exposing boxers with Perry's face printed on them, another jab at their Foe Yay-type relationship.
In "Monsters from the Id", Doofenshmirtz creates an "Underwear-Inator" to put ratty, embarrassing underwear on anyone shot by it. It usually does briefs, but it does give Norm the Giant Robot Man a pair of white boxer shorts with red polka dots.
In Ed, Edd n Eddy when the Kankers are tearing apart Eddy's room Marie pulls out a pair of underwear.
Marie: Hey are these real leopard-skin undies? (Cartoonish stretching sounds)
Edd: That's quite a statement, Eddy.
Eddy: (pulling at his shirt collar with steam coming from it) They're my brother's. What?
Marie: Says "Eddy" on the tag.
Eddy usually wears polka dot boxers
The Aristocats: Edgar the butler is revealed to be wearing red and white striped boxers twice in the film, the first being the scene where he is hearing Madame tell her lawyer about the fact that when she dies, her property will be handed down to her cats, and when they die, then it's Edgar that will be owning her property. Edgar however, thinks that the cats will live longer than him, and as a result, he starts to come up with plans to get rid of said cats. The second scene where Edgar's underwear is seen is when a huge chunk of his pants is torn off by a pair of dogs.
Linguini from Ratatouille is revealed to be wearing Incredibles print boxers during the scene where he is trying to hide Remy the rat from the other chefs.
In The Thief and the Cobbler, Zigzag is wearing pink boxers with purple stylized "Z"s under his robe when he launches himself up through the floor of the throne room.
In the Looney Tunes short "The Rabbit of Seville" while dressed as a woman and performing a dance with scissors he cuts off Elmer Fudd's overall straps revealing that he is wearing red boxers with white flowers on them.
In the episode "Waterboy Bogus", Mole is shown to be wearing a pair of white boxers with pink polka dots on them underneath the mermaid costume that he wore, despite the fact that he doesn't normally wear such undergarments.
Bogus is revealed to be wearing a pair of pink boxer shorts with white polka dots on them when Brattus pulls down his pants in the first act of the episode "Battle Action Bogus".
Bogus is also briefly shown to be wearing a pair of lavender boxers with white polka dots in the episode "The Bogus Invasion".
Tubby is shown to be wearing a pair of white boxers with red polka dots on them in The Little Lulu Show episode "Bowlfinger", after Iggy pulls off his pants by accident.
Code Lyoko: Jeremie Belpois is revealed to wear Kangaroo Boxer shorts.
Punky Brewster: In "The Quartersize Quarterback," Allen bumbles as he attempts to catch a football pass. His belt snaps and his trousers drop, revealing polka dot boxers.
The Looney Tunes Show: In "SuperRabbit", Superabbit (Bugs) rips a suit of powered armour off Lex Luthor (Elmer Fudd) to reveal that Lex is wearing boxers with the Superman symbol on them.
As revealed at the end of ''Tokyo Mater'', unpainted metal is actually considered to be the automobile equivalent of underwear.
In one episode of Producing Parker, Simon dances in a pair of space themed underwear.
Jimmy from Ed, Edd n Eddy wears briefs that have either rainbows or teddy bears on them. This fits his overly cute personality.
An example of incredible bas taste in the "Revolution" episode of Teen Titans; at the end of the episode, Mad Mod frees himself from Robin's grasp by wriggling out of his pants, revealing briefs with the Union Jack printed on them.
Childish women's underwear
Rita from Flushed Away is revealed to have blue-and-white striped panties during the scene where Roddy accidentally pulls down her pants while escaping from the Toad and his henchrats while using the sewer gate's main cable as a zipline. Rita then proceeds to use said cable as a belt shortly after escaping.
On Total Drama Island, Heather fixes Gwen's pants during a mountain climbing challenge so she could rip them off when she was halfway up the cliff and reveal the Perky Goth purple skull panties to the world.
In The Replacements episode Best Friends For-Never we see a flashback to when Riley and Tasumi first met. The Japanese girl was hiding in the closet because she split her armor. Underneath it she was wearing aquamarine panties with pinks hearts on them. Riley fixes it with duct tape.
In the Ed, Edd n Eddy episode “Boom Boom Out Goes The Ed” Nazz spends the entire episode wearing frilly purple panties with pink hearts on them. At least it was more dignified than what Jimmy was wearing.