The Diablo Sauce in Kim Possible, which Ron uses packets of for hand warmers at one point. They're spicy enough to fry a robot's circuits.
The Fire Salt in Steven Universe, that acts as a very hot sauce, consuming a full bottle causes anyone to literally breath out fire.
The Merciless Hot Pepper of Quetzalsacatenango, also known as the Guatemalan Insanity Pepper from The Simpsons. They're "grown deep in the jungle primeval by the inmates of a Guatemalan insane asylum." Homer has to lacquer his mouth and esophagus in order to eat one, and is still driven temporarily insane by its spiciness.
Hermes from Futurama, in the episode "The Six Million Dollar Mon", spilled a drop of sauce from his wife's spicy curry goat, and it ate its way through the dining-room table, the floor, two other residences underneath, and on down to Robot Hell, where it scalded the Robot Devil. Hermes himself thought it needed more hot sauce.
Fortunately, this ends up saving him later on in the episode; years of his wife's overly spicy cooking somehow made his skin extremely spicy, and when Ax-Crazy robot Roberto eats a piece... he melts.
This one deserves expanding a bit. When in Mexico, our Anti Heroes decide to try the local cuisine. While waiting, they torment a dog by force-feeding it the bottle of "Killer Hot Sauce" on their table. This pisses off the Mexican cook, who reaches for the "Mexican Death Sauce" (which is in a bottle with smoke coming out of it) and puts it on their tacos. Cue Fire-Breathing Diner. (This scene even provides the page image for that trope.)
The "Pure Jalapini Concentrate" Pete tried to use on Goofy in the 2-part opening of Goof Troop, though it only succeeded in setting Pete's mouth on fire.
On Sponge Bob Square Pants, Sandy threatens SpongeBob with a single drop of hot sauce to the tongue. A drop of hot sauce that talks.
"By the powers of naughtiness, I command this particular drop of hot sauce to be really, really hot!"
After SpongeBob surrenders, she casually starts gulping the stuff down like it was lemonade.
As very spicy mustard (which features a different chemical than capsaicin to produce its spice) used to be much more common, some older works feature this instead of the modern hot sauce. Tom and Jerry features this in several episodes.
You can get very hot mustard as a condiment at your average Chinese take-out restaurant. WARNING: Do NOT eat more than a light glaze of this stuff on your food - unless you have a cold and you really want to clear out your sinuses.
Very hot mustard is also commonly found in Russian cuisine. In Russia, you can buy this stuff in quite large jars cheaply.
In My Little Pony: Friendship Is Magic, Pinkie Pie apparently loves this. She likes to put it on cupcakes. Her brand is so hot that, when Twilight Sparkle accidentally drank some, her tail and mane caught on fire.
Pinkie Pie can't handle the spiciness of liquid rainbow (Yes, literal rainbows), however. She even breathes fire in several different colors!
In the Hey Arnold! episode "On The Lam", Harold, Stinky, and Sid launch a rocket filled with vinegar, baking soda, and Stinky's bottle of Louisiana Hot Sauce, which they believed to blow up an old police station with, when it was really demolished by Ernie.
The "Honor Thy Ed" episode of Ed, Edd n Eddy starts with the Eds having set up a taco stand selling "tacos" made of paper plates, grass, and shredded crayons. After Ed, Extreme Omnivore he is, eats several crayons thinking they are flavors, Edd humors him and takes a bite of one. He is disgusted and needs water, but the closest thing is Eddy's Brother's Armenian Secret Hot Sauce. He guzzles part of it and seems unfazed at first. Ed also consumes some. Eddy thinks the sauce didn't work and also eats some. It's at that moment the effect takes hold, however. We see a Discretion Shot from down the street as a tower of flame erupts from the stand and the Eds sticking their mouths on a sprinkler.
"Grandpappy's Texas Tin-Horn Sizzlin' Salsa Sauce" in Xiaolin Showdown. Two bottles of it turn Dojo, a dragon who can normally only breath soot clouds, into a volcano.
Chowder gets a whole episode based around this, considering it's a cartoon about food. The episode (aptly named "the Fire Breather") introduces the rarest and hottest peppers in the world, the Dancing Diablos. Peppers so hot that it turns your stomach into a furnace and makes you breath fire every time you open your mouth. Guess who decides to eat all of them?
Garfield had Grandma's sausage gravy in A Garfield Christmas. It started out as Mom's normal gravy, but Grandma snuck in and added chili powder while whispering about how her gravy won first prize at the fair three years in a row while Mom's didn't even place. A bit later, Garfield comes in and gets some on his finger, then licks it. Cue the fire breath a second later, and then Garfield going "Perfect!".
In Garfield and Friends, we see him using this on a horse named Certain Death.
Regular Show: When Mordecai and Rigby try to pass off an unconscious Benson as awake, they get him involved in a spicy food contest with a guy. Benson comes to just as the guy prepares a concoction he calls "Mississippi Queen", so he has Mordecai and Rigby drink it as well. At first they feel nothing, but suddenly start hallucinating and wake up the next morning in the park.
Taz-Mania: In "Francis Takes a Stand", Francis switches Taz's lemonade recipe with a recipe for a Blazing Inferno Hellfire Sauce. Naturally, Francis ends up drinking more of the stuff than Taz does.
Grossology: Pink-Eye attempts to torture Lance Boil by force feeding 10 bottles of this stuff. It fails because Boil has replaced his stomach with an artificial digestive system.
Storm Hawks gives us Skyside Shanty's signature extra potent, quadruple malt vinegar.
Junko: Kinda sour, but I like it.
Finn: What a shock.
The super-hot chili peppers that prove to to be the best defence against the mutant raccoons in the Camp Lakebottom episode "High Plains Garbage Eater".
In The Smurfs episode "Gingerbread Smurfs", Brainy has Clumsy add volcanic lava juice to his recipe for making gingerbread cookies to make them really spicy. It is so powerful that it blows up the kitchen when it is added.
In an episode of Goof Troop, Pete tried to prank Goofy with the spiciest sandwich EVER. He loaded it up with normal hot sauce and spicy ingredients, and added a drop of Hellfire Sauce that made the sandwich boil. Goofy ate it without flinching and asked for some hot sauce. Pete, in a state of shocked fury, took a big bite of the sandwich, turned red, ran outside and drank his entire swimming pool.