Reis in Final Fantasy Tactics, being basically a Dragon in the shape of a woman, can breathe Fire, Ice, or Electricity. Oh, and Holy. However that works. This may not have been clear in the original translation (Where breath gets translated Braclet)...
All Final Fantasy games where Blue Mages appear feature them learning breath weapons. Usually, three are available: Aqua Breath, Fire Breath and Bad Breath (Malboro's famous chemical weapon grade halitosis attack).
Barthandelus, Big Bad and Master Troll of Final Fantasy XIII, has two ultimate moves named "Thanatosian Smile" and "Thanatosian Laughter," both of which consist of him barfing lasers on you.
Sigma at the end of Mega Man X6, as well as too many other robots to sanely list here.
In V for the Game Boy, Mega Man has spent the game using the Mega Arm, because his Super Mega Buster can't affect this batch of robots. What happened to it, you ask? Dr. Light put it in Rush. He shoots the charged shots from his mouth (having no arms).
Virtually every weapon in Mega Man Star Force that does not simply come into existence in midair is fired from Omega-Xis' mouth. As a result, most of what Mega Man fires is technically a Breath Weapon, most notably the first game's Tail Burner, which is actual fire. Well, digital fire.
Breathing fire is one of Bowser's trademark characteristics in the Super Mario Bros.. series, but there are a few other critters that have this ability (such as the Dino Rhinos and Reznors in Super Mario World).
The Dragon and Dragon Zombie species predictably have a breath weapon, using fire (Or wind or ice in the first game) and darkness respectively. There's also the Cockatrice, who uses either petrifying breath or miasma depending on the game.
The two-headed dragons from the fifth entry can breathe fire and ice simultaneously, one per head. Also, Berserk Usalia breathes either lasers or energy balls.
The Pandaren Brewmasters from Warcraft III can breathe "fire" (ignited grain alcohol) as part of their standard skills, although they are firmly good guys. This also shows up on some of the neutral monsters. There is also a breath of frost, but it shows up much more rarely.
Dragons in the same games also have breath attacks. Red dragons breathe fire, black ones breath molten lava, blue ones breath ice or magic energy, green ones acid and in World of Warcraft bronze ones breathe magic, superheated sand that causes rapid ageing, although in Warcraft 3 it's clearly lightning.
In World of Warcraft, almost all Dragon-type bosses, in addition to regular bolts and fire/frost/poison/whatever else breath have a devastating attack that begins with: "X takes a deep breath..." and generally must be avoided via some sort of trick. Black dragon Onyxia is mostly infamous for her insanely powerful Deep Breath which she uses completely randomly, which makes it hard to avoid. For months, if not years, people have tried to come up with explanations of what makes Onyxia Deep Breath more. Some theories make more sense than others, but all are false. It's really random.
Ironically Onyxia is also a subversion of this trope, since her Deep Breath flames seem to come straight from the ground instead of her mouth, perhaps due to graphic limitations?
Chimeras have one head that spits acidic bile and another that breath's magical lightning.
Hunter-pet Chimeras in WoW breathe frost and lightning. Even if the non-tamed mob has a poison/acid ability as a mob (which it loses when tamed).
Frost Wyrms have an ice breath weapon.
Though, since Frost Wyrms are giant reanimated dragon skeletons, its debatable if this can be called a "breath" weapon (what with them not having actual lungs and all).
In Mists of Pandaria, the Monk Brewmaster spec (which is the tank spec) has the fire-breathing ability mentioned above as one of their area-of-effect aggro-generating methods that also causes damage over time. Especially effective when done after dousing the enemies with alcohol using the keg toss.
Beginning in his first game, Kirby could spit puffs of air with enough force to damage most enemies, and power-ups allowed him to spit puffs of minty-fresh or spicy-hot breath. In later games, after he learned to steal enemy powers, he could gain the ability to exhale fire or ice.
The Chroma Warframe of Warframe has the ability to breathe out whatever element the player has chosen, ranging from fire to ice to acid to even electricity.
Spyro the Dragon isn't limited to merely fire breath - he goes as far as to launch ice, earth chunks and electricity out of his mouth. Cynder uses shadow, wind, poison and fear.
He can only launch chunks of earth if he scoops them up in his mouth beforehand. It's not a breath weapon, it's just spitting.
The daedroth and spider daedra in The Elder Scrolls IV: Oblivion. The daedroth spits balls of fire; the spider daedra breathes lightning, and also casts a touch-range paralysis spell by exhaling green haze into its palm and then slapping the target.
And let's not forget those ever-annoying Freezzards in the Ice Cave...
Scorpion from the Mortal Kombat series can pull off his ninja hood to breathe fire, usually as a Fatality.
Jak 3: Wastelander's Dark Makers have access to giant stompy Humongous MechaSpider Tanks. How could you make this kind of combo even worse for Jak? The head can fire a laser from where a mouth would be on an organic creature.
In Lost Souls MUD, a number of species breathe fire by way of phlogiston glands. Esoteric breath weapons are explained by phlogiston absorbing magical essences that infuse the glands' owner.
The Giant Minotaur boss in God of War had a fiery breath attack.
Fire Emblem dragons, both manaketes and the laguz, attack with their breath. Sometimes, there are even multiple different types of breath for dragons to use, based on either the species of the dragons themselves or the dragonstones they possess.
The boss Fraaz from Spirit Tracks is an entire boss fight centered around its Breath Weapon. The boss inflates with a specific element (fire or "icy fire") and the player has to use the polar opposite against Fraaz as it's inflating with another attack.
Breath attacks form a skill family in Dragon Quest. The weaker versions are pretty common among low- to mid-level monsters, while the most powerful breath attacks tend to be spammed by bosses. (Especially the Final Boss.) Later games allow the player to recruit monsters or use special character classes that have access to these abilities. What makes them particularly obnoxious is that despite dealing elemental damage, they don't count as magic attacks, so the only ways to protect yourself from them are with equipment, spells or skills which either reduce damage taken from those elements regardless of the nature of the attack, or specifically defend against breath attacks.
Monster Hunter used basically every Wyvern, Dragon, or "Boss" monster plus a few minor enemies. Yian Kut-Ku, Rathalos/Rathian, Yian Garuga, and if you want to be technical, a King Shakalaka all have a fire-based breath attack. Mostly fireballs, and in the case of the Yian Kut-Ku, it is simply lobbed out. The Gravios and (more rarely) Basarios have a beam-like weapon that also has an Exhaust after-effect, where they are likely to burn everything within melee range of them after disintegrating you with their throat laser. Among the -prey series there is Giaprey and Ioprey which spit ice and poison based attack respectively. Hypnoctrice has Sleep- based attacks. Congalala can eat mushrooms to belch out fire, poison gas, or paralyzing gas. The Elder Dragon Teostra has a flamethrower-esque attack.
Tri ups the ante considerably. Rathalos and Rathian still spit fireballs, but Rathian can spit three at once or a "blaster" which blows up everything a good ways in front of it. There's also the Qurupeco, which spits an oil-like substance as one of its attacks; the Royal Ludroth, which spits water; the Gigginox with poison; the Baggi and Great Baggi with sleeping agents. Up until now, it was sensible. But then there's the Lagiacrus, which shoots ball lightning from its gaping maw, the Barioth ejects a miniature blizzard as its breath weapon, and the Agnaktor fires what is essentially a heat beam!
Which doesn't actually mean it ups the ante, as many monsters in previous games could also do things like that. Khezu could spit electricity, and Gravios's sweeping heat beam was absolutely infamous.
Ceadeus launches a powerful stream of water and Alatreon launches ice breath
Battle for Wesnoth's Drakes are all capable of breathing fire although one caste (the Clasher unit line) gives up this ability in favour of the protection provided by heavy armour, which restricts their breathing enough to prevent this.
Among the power-ups in Gauntlet Legends and its remake Gauntlet: Dark Legacy are breath weapons, including Fire Breath, Acid Breath, and Lightning Breath. Unlike most of the power-ups, these have ammo-based usage instead of being active for a limited time. A lot of the bosses use breath attacks too. Even though there's no Light Breath power-up but there's a Light Amulet, Skorne can spit beams of light at you.
Pokémon features a few moves like this, namely Dragon Breath and Frost Breath. The anime expands on this making many attacks mouth-based.
The final boss of Mass Effect 2 is a gigantic partially-complete Human Reaper that attacks with a gigantic, long-charging beam attack from its mouth. It's also its own worst enemy with the thing, often sweeping its allies off the platform for you.
Common in Dark Souls; it's a universal trait of dragons and drakes, but other enemies can do it too. Fire breath is most common, but some drakes and gargoyles are shown "breathing" lightning, hydras launch giant water balls, undead dragons spew toxins, basilisks emit clouds that curse/petrify people, and Seath has sorcery-infused curse-breath that also causes crystals to spontaneously pop up where it hits. The player can even gain the ability to breathe fire if they follow the Path of the Dragon.
In League of Legends Kog'Maw's attacks all involve spewing digestive fluids at his enemies. Shyvana also breathes fire when in her dragon form.
NetHack has dragons of nine different colours, covering each of eight different breath attacks (silver and white dragons both use cold breath). Hell hounds, winter wolves, iron golems and Nazgul can also breathe (fire, cold, poison and sleep respectively); and then there's the Chromatic Dragon.
Alisia Dragoon uses it for several things, including Dragon Frye's fire, the fireballs spat by the Stage 4 boss, not to mention the various things such as sparks and worms that come out of Baldour's mouth.
The dragon in Dra Koi fires mouth beams as both a dragon and in human form, at which point her eyes glow and she grows huge fangs.
In Pikmin the enemies Fiery Blowhogs and Watery Blowhogs breath fire and water respectively.
Higher-level skags in Borderlands have fire and laser breath, as well as spitting shock and acid.
In Borderlands 2 certain human enemies have fire breath as well as being able to spew acid and slag at enemies. Krieg can breathe fire with the Hellfire Halitosis ability.
Awakening: The Sunhook Spire introduces special dragon altars for Sophia's pet pocket dragon that can allow it to exchange its normal fire-breathing for other elemental powers through its breath.
Ryoufu from World Heroes can take a chug of alcohol and then spit fire on his opponents.
In the Golden Axe games, both the heroes and the villains in the game can trigger fire breaths when riding aboard dragons. Red dragons can shoot fire balls that can attack from a distance, while blue dragons can shoot bursts of fire that can attack enemies close to them.
In the Streets of Rage games, the Big Ben enemy can breathe fire on the players.
In Quackshot, Shere Khan from The Jungle Book appears as a boss in the Maharajah level. His basic attack is breathing fire at Donald, which is ironic, because in The Jungle Book movie, fire is his one weakness, and towards the end of that movie, Mowgli defeated him by tying a burning branch to his tail.
The Dark Green Baby Yoshis are sprite-swaps of the Dino Torches and have the same behavior as them.
The Sakura Tori is a pink bird and it can spit fireballs at its foes.
In Dwarf Fortress, dragons have naturally breathed fire from the very earliest versions, but in more recent updates underground creatures have also gained breath attacks that can cause disease, flesh rot, and overall unpleasant deaths. The game can also be modded so that practically any effect can be used in a breath attack, from boiling gold to transformation.
In Activision's Oink!, the Big Bad Wolf uses his breath to blow away the bricks of the Three Little Pigs' houses, which if he manages to blow a big enough hole can turn into a Tractor Beam if it touches one of the pigs.
Sonic the Hedgehog began having bosses with these starting with the 3-D games. Perfect Chaos has a powerful beam he fires from his mouth strong enough to destroy the Egg Carrier 2 with one blast. Iblis breathes fire in his first form. Nega-Wisp Armor spits cubes and spikes from its main mouth and two hand-mouths and lasers from the hand-mouths.
Pony Island: One of the first things you get to do is to enable "Pony Lasers" that enable your pony to fire a Wave Motion Gun from its mouth.
Most of the monsters in Evolve have one of some kind. Goliath has standard fire breath, Kraken launches a blast of lightning and dark energy, Behemoth spits lumps of lava, and Gorgon sprays acid.