In Dragon Ball Z, Super Saiyans get a lot of use out of this trope, but play it closer to it's Truth in Television version. They do gain spikier hair and begin glowing vibrant colors & in addition to this they get a massive power up. Some later upgrades even include large muscles, hair and even eyebrows.
In Bleach, the Vizards can put their Hollow masks on as a definite Game Face.
In Fullmetal Alchemist: Brotherhood, after Selim Bradley reveals his true identity as Pride, he's consistently drawn with sharper features and more menacing eyes instead of the almost chibi appearance he had before.
Both incarnations of Greed also had a charming, handsome human face (granted, one had pointed teeth), which in a serious fight, would be covered by the demonic, armored face. Being rather vain, Greed really hated using that form unless he really had to.
Rosario + Vampire: Absolutely everywhere, given that all of the monsters in it can take a perfect human disguise form, and given that a Global Masquerade is firmly in effect. Succubi, Yuki'onna and Gorgons are all good examples of slighter changes (hair turning to ice, lengthening nails and ears or hair turning to snakes) while a great many other monsters show much more drastic change to their true forms. Of course while they are seldom angered enough to do so, the main characters have all shown ability to pull some very scary game faces despite their good natures.
Death Note: Light Yagami does this, having hidden his true personality almost to the very end, he loses what's left of his sanity when he's exposed, and breaks downand confesses, as he'd just got done swearing he'd never do. The scary red eyes are just for us, the audience - but the frightening facial contortions, dramatic gestures and visible madness? Yeah, they're free for all. What makes it a Game Face is that it liberates him to finally state his case in person. Not that it convinces anyone.
What occasionally happens to Haruko's face in FLCL during fights could be described as this. She gets huge, alien-like eyes and sharp teeth, which may or nay be closer to her original alien appearance.
In Fairy Tail, whenever he gets angry, Natsu has two veins that bulge in a diagonal slant above his eyebrows and his eyes change in to pale yellowish color with slit-like pupils.
A Full-Body Take Over gives Elfman and Mirajane this effect.
Inverted and subverted in Queen's Blade with The Swamp Witch, at least in the anime: Her ghostly looks is the real face of the spirit who controls Werbellia, a demon princess she's using for doing all her atrocities.
In Excalibur, Rachel Summers (avatar of thePhoenix Force) gets scary-looking lines on her face when she gets serious. They're actually scars that are always there, and she uses her telepathy to make others see an undamaged face. When she has to use her full power for butt-kicking, she can't spare any for the illusion and lets it drop. Or sometimes she's just too pissed off to concentrate on it.
Marvel Comics' nearly-omnipotent pseudo-villain Molecule Man has something (very) similar: a pattern of lightning-shaped scars (caused by the accident which gave him his powers) across his face which seem to appear or disappear when he is either relaxed and mentally balanced, or furious and/or about to break down and cry. As the scars also make his lips appear serrated, it is a pretty creepy game face considering he normally looks rather nerdy.
In the Brazilian film O Auto da Compadecida, Satan stops using his human face and goes into a devilish face (as well as a deep voice) after the protagonist angers him enough.
The Lost Boys: A seemingly-innocuous character (Edward Herrmann) turns out to be the Big Bad and we don't know it until he gets his Game Face — but the effect may seem doubly disconcerting since the original actor was chosen to look especially nonthreatening. James Spader in Wolf is similar.
Beetlejuice: If you ever meet Beetlejuice, be sure to ask him "Can you be scary?"
Senator Palpatine/Darth Sidious in Star Wars: Revenge of the Sith, after using his Force Lightning. There is some debate over whether this is a "Game Face" revealed by him finally letting his more "conventional" appearance slip and throwing himself completely to the Dark Side, or whether it is merely scarring as a result of high-potency Sith Lightning being reflected back at him at close range. Either way, the trope fits as a story-telling device - his face-change IS the point where everything takes a seriously dark turn.
Played for laughs in Galaxy Quest, where the crew encounters some weird baby-looking aliens on a desert planet. One of the cute aliens gets hurt, and the others crowd around it as if to help it get water... only to reveal demonic faces with terrifyingly sharp teeth and devour the thing alive.
In Blood And Donuts, Boya uses his game face to intimidate the gangsters that are harassing him and Earl. Unsurprisingly, it works very well.
In Nightbreed (pictured above), Lori and her boyfriend Eric Boone, now a newly-turned member of the Nightbreed, have escaped Midian and are on the run from the police. When the cops marshal an entire force to arrest him he turns around to show Lori his true face.
Needful Things: Leland Gaunt. He looks like a charming and handsome man, but he's actually a demon with claws, and a face that is "a horror of eyes and teeth".
The trolls from Twilight Eyes. They can switch between horror and looking like ordinary humans at will, and to detect them while they look like humans, you need twilight eyes.
Vampires in The Hollows series eyes go completely black and their fangs extend when overcome by bloodlust.
Red Court vampires are monstrous, bat-winged demons wearing human skins. When things get hairy, the skins come off.
Harry himself refers to Will and Georgia's shapeshifting into their wolf forms as putting on the game face.
White Court Vampires look beautiful and pale and when they go Game Face, it's the same, but with a distinctly inhuman bent to it.
To be specific, they usually look like pale and beautiful humans with silver-grey eyes. When they get serious, they become inhumanly pale, their eyes glow silvery white and their beauty becomes terrifying as well as alluring. During the one occasion a White Court vampire used her full powers, her entire body glowed silver and Harry himself barely resisted throwing himself at her feet, never mind the actual target of her...abilities.
Denarians, Nicodemus aside, each have a personal Game Face.
According to Murphy in Aftermath, Harry is normally this awkward, geeky, weirdly-dressed smartass. When things get dangerous, he becomes a dark, commanding, imperious figure in the center of both figurative and literal firestorms. Badass doesn't begin to cover it.
It was revealed that if you ever see pale, lovely Mab with her beautiful green eyes and bright, winter-themed dresses get so pissed off her hair, eyes and clothes turn pure black, with black veins crisscrossing her skin, you better start praying to whatever god(s) you believe in that she spares your life- or at least kills you painlessly.
Wolfgang's pack that, while they find it convenient to hunt in wolf form, are not above frightening peasants.
Elves without their glamours on are also quite frightening. Especially the queen.
Usually the elves seem less impressive with their glamour off, usually described as small, pathetic, dull and gray. There's a deliberate parallel to The Grays there.
Death himself, in Mort, when out looking for a new job, has to deal with a clerk who is slightly condescending and who not-so-subtly tells Death that he has no useful skills. Death then forcibly overrides the man's Weirdness Censor for only a fraction of a second, allowing the man to see Death as wizards and cats are able to. Death then has to deal with a customer who had arrived to complain, eventually giving up and bribing her.
Speaking of Death, Mort and Susan have a version of this (Mort while he was playing Death's role and Susan on a permanent basis on account of being Death's granddaughter. This involves switching from normal speech to doing The Voice and talking like this and something frightening happens with their face (probably their skull shows through). It tends to scare most people.
The Second Apocalypse has Kellhus, a genius with literally no emotions. However, he has complete control over his entire body, including his facial muscles, so he is able to fake emotions without any difficulty. When someone finally figures out that he's been manipulating everyone the entire time and confronts him about it, every muscle in his face goes dead, like turning off a light.
Subverted in The Jennifer Morgue by Charles Stross. The protagonist is working with a succubus covered by a level-three glamour that makes her look like a gorgeous woman. Eventually despite her warnings he insists that she reveal her real face — instead of the hideous demon he expects, she turns out she's a Half-Human Hybrid that he actually finds rather attractive.
A non-evil variant in Akata Witch by Nnedi Okorafor. In it, all Leopard People (people with special magical abilities) have a "spirit face", that is almost like their true form. Seeing another person's spirit face is seen as almost the same as seeing them naked. However, Sunny plays this straight when she gets in a fight with another student and shows them her spirit face to scare them.
Once the shit has well and truly hit the fan in The Shining, the Overlook forces Jack Torrence to smash his face in with a roque mallet, leaving him a faceless meat puppet of the hotel's malevolent spirits.
Vampires have a human face, and a demonic face. The demonic face has vaguely bat-like bumps along their foreheads, yellow eyes, and of course, fangs. Some vampires were Shapeshifter Mode Locked into their demon face, such as The Master and Kakistos, presumably because of their extreme age (the Master being identified as the oldest known vampire at the time of his introduction).
It's implied that their demon face is stronger, given Buffy told Angel to put his Game Face on during the Season One Finale, and it's confirmed in the spin-off (see below).
On the flip side, there was Dracula, who had only a human face — but had perpetual fangs. Spike said it was due to gypsy magic, which was the handwave used to explain all the ways Dracula differed from other Buffyverse vampires.
The Anointed One was one of the few recurring vampires to never show his, leaving it ambiguous as to whether or not it would've been Nightmare Retardant or just silly.
Vengeance demons as well.
Willow has two game faces, her lesser used "Light Willow" look and her more well-known "Dark Willow" persona that comes complete with black eyes, black hair, and evil face veins.
The vampires. Angel even kept it when turned into a muppet. One episode states that he gets stronger with his Game Face, but his bloodlust also increases.
Vampires' demonic faces become much worse (and the advantages and drawbacks both get amped up) in the dimension of Pylea, due to its unique metaphysical properties causing hybrid species such as Vampires to manifest as either fully human or fully demon. When Angel changed his face during a fight, he ended up turning into a ferocious demon called a "Vantal", with green skin, red eyes, and horns. His strength and durability massively increased, but he could barely control his bloodlust in this form and nearly killed all his friends, and when he finally changes back to his human form, he's horrified. It's heavily implied that the Vantal is the true face of the demon that dwells within all vampires, and the same one responsible for creating their race by infecting dead humans, according to Giles.
Doyle had one too; his demon form has green skin and is covered in spikes. He's stronger in demon form, but prefers his human form, but this is because he likes to pass for (relatively) human, not due to bloodlust. Played for humor in that Doyle would involuntarily shift to his demon form (though only for a moment) when he sneezed.
Also Jasmine, with her maggoty, revolting true face and her beautiful false one.
Vampires in Being Human normally look completely human, but when about to bite people or otherwise get violent, their eyes turn solid black and their fangs come out. They're also able to consciously activate this when they aren't being violent, but still want to scare people.
In Kamen Rider Faiz, an Orphnoch will typically show a shadowy version of its transformed state over its face as a way of saying "don't screw with me." This isn't the actual battle mode, though (that would be a full transformation into the rubber suit, which starts with the 'game face' appearing.)
Kamen Rider Kiva: The red and blue vein-like lines on a Fangire's neck and lower face before transforming.
Kamen Rider Wizard: The Phantoms do a combination of both the Orphnochs' and Fangires' Game Faces (a shadowy aura of the monster face over their human forms before turning into their true forms).
Ironically, Kamen Rider Agito had a heroic variation. Another Agito would proceed his Rider Kick by retracting his mouthplate to reveal a skeletal mouth underneath.
In the episodes "The Impossible Planet"/"The Satan Pit" of Doctor Who, a person who has been taken over by the alien of the week will have red eyes (red eyes only for the Ood) and mysterious writing tattooed on their skin (including their faces) as well as an I-will-hurt-you expression in general, complete with narrowed eyes and a mischievous grin.
Moonlight vampires extend fangs and morph into slightly-beastly faces in a split-second. When dehydrated, their eyes turn yellow.
Henry Fitzroy in Blood Ties extends fangs, and his eyes go black. The eyes also go black during hypnosis.
Vampires in Sanctuary gain slanted black eyes, long talons on their fingers, a full mouth full of fangs and deeper voices when they shift into vampire mode. Tesla's skin also became grey on the first occassion he shifted, but this was largely ignored during later transformations.
In the Lost in Space episode “The Golden Man” (Season 2, Episode 15), Mr. Keema proposes to Dr. Smith that in exchange for the weapons from the Jupiter 2 to defeat his enemy the frog faced alien, he will transport Doctor Smith back to Earth. Dr. Smith brings the weapons without permission. As he is talking to the handsome alien about extending the hand of friendship, Mr. Keema transforms himself into his twisted ugly self and then tells Smith that after he disposes of the frog alien, he intends to destroy the earthlings.
Charmed has half demon-half human hybrids able to do this. Cole has a human form but has to embrace his demon self fully if he wants to tap into his powers. The demon self comes complete with red skin and black tribal markings. One Monster of the Week has the same appearance in demon form, but with the colors inverted.
All Wesen in Grimm have this, which is how they are recognized by Grimms and each other. Normal (non-Grimm) humans still can't see it unless the Wesen want them to. The latter is called "woging" (pronounced with a "v") from German "woge" meaning "wave". Interestingly, while Grimms don't have a Game Face, their obvious reaction to a Wesen briefly showing theirs is what gives the Grimms away (the Wesen know when they're woging).
The last part was RetConned into the Grimms having Black Eyes of Evil that can only be seen by a woged Wesen. The eyes reflect the Wesen's true nature back at it, although sunglasses block the effect.
An episode deals with the side effects of a Wesen being forced to go "full woge" (i.e. being seen by all) frequently, eventually causing the Wesen to start losing control over his Wesen nature and occasionally going into "full woge" without meaning to and amping out his aggression Up to Eleven.
The Irish folk hero Cuchulain had his "warp spasm," which is lovingly and nauseatingly described in Táin Bó Cúailnge. You can read the relevant part here.
Werewolves and other shapeshifters have several Game Faces somewhere between man and their animal form, up to the ultimate Crinos (or Gauru, or whatever) man-wolf (or whatever half-animal) form. But in Werewolf: The Apocalypse they can't display the Game Face to ordinary humans — there's a thing called Delirium, which instinctively makes the human explain away or forget what they saw, and potentially go a little insane as well.
Vampires can extend their fangs at will, but, depending on their Discipline set, can turn into wolves, hideous monstrosities, or even snakes. It has been speculated that turning into a snake never helps. The most common "Game Face" among the Kindred is the Presence power "Dread Gaze," which serves to terrify the hell out of most of the mortals that vampires encounter.
Kindred of the East, the East Asian counterparts to western vampires, have even more and increasingly bizarre demon forms.
Fomori, being corrupt nature spirits inhabiting human bodies, can take a Merit that allows them to hide their powers and putrescence, but generally they are always in their Game Face.
Demons all have Apocalyptic Forms that resemble either angels or demons, depending on how Tormented the demon in question is. The transformation is instantaneous, but might not happen at all if witnessing humans' disbelief is strong enough.
Mages can draw on the Supernal Realm to increase their Aura and cause an effect like this. The precise appearance varies depending on the Mage's Path. Thyrsus (Shamans) make everything around them seem alive, while Mastigos (Warlocks) can make things go dark and sound like howling demons.
Changelings can burn all their Glamour at once in order to drop the Mask and reveal what they truly look like. This is not a recommended course of action, as now you're all out of magic juice, people will likely think you're a monster, and doing so risks damaging the Sanity Meter. The only real feasible use of this tactic is if you're a low-Clarity Autumn Court member who then proceeds to feed on their tasty, tasty fear. There are, however, Contracts that will allow a similar effect without risking your mental stability.
Vampires with the Nightmare Discipline have Game Face as a starting power - Monstrous Countenance, which allows you to unleash the full and terrible visage of the Beast. Most mortals flee on sight.
Demons can assume their true forms at will. Like their oWOD counterparts, they resemble angels or demons - though it's not dependent on their psyche - and add biomechanical horror to the mix as well. However, doing so is risky, as it may draw the attention of their former master, the God-Machine.
Feng Shui's monsters all have a horrific appearance, and monsters (both PCs and bad guys) can choose to take a Creature Power called "Brain Shredder," which basically provides one of these and weaponizes it to cause damage through fear.
The Dawn Caste Solar anima power in Exalted includes an effect of this sort. It causes the Solar to appear larger, more glorious, and terrifying with Glowing Eyes of Doom and a fully active Battle Aura. Interestingly, this particular Game Face can be both activated deliberately or go off on its own simply through expending enough power in combat.
The Dusk Caste Abyssals and Slayer Caste Infernals, as counterparts to the Dawns, have a similar power.
Dungeons & Dragons 3.5th Edition had Pale Night, an ancient demon/Eldritch Abomination. She's apparently so evil that reality itself has to shield itself from her. Evil enough that her strongest attack is to show her face, instantly killing all but the strongest-willed, and even those that survive do so by not comprehending her true form.
Shifters, from the Eberron campaign setting, have a small touch of lycanthrope heritage. For a brief time each day, they can "shift" into a slightly bestial but physically more powerful form.
D&D also has a monster called a Krenshar that looks like a type of great cat, but it has very loose skin around its face. Thus, when it wants to scare off predators, is turns its face inside out.
Lahmian- and Von Carstein-clan Vampires in Warhammer Fantasy are often said to be extremely beautiful and/or handsome. You would never tell this by the models, because they're always sculpted with their Game Faces on.
The killer in Persona 4 does this when you've figured out who he is. The change in his expression as he gleefully and self-righteously reveals his true personality is... unnerving, to say the least.
Several of the villains in Final Fantasy Tactics transform into their Lucavi forms for the final confrontation against you. Oddly, the demon forms are often easier to beat than the human ones, particularly Elmdore and his Assassins.
In the latest World of Warcraft expansion, the Worgen shift into animalistic form whenever they enter combat, including a permanent scowl.
Scarfy in the Kirby games is a cute little flying critter that changes into a furious, one-eyed Action Bomb.
Matt Engarde, the killer in the last case from Phoenix Wright: Ace Attorney: Justice For All, looks nice enough... but when he drops the act, he flips his hair, revealing the scars over his left eye, and his expression becomes a perpetual sneer.
Tekken's Angel has one in the second Tag game. If you see◊ it you'd think she's about to melt your face.
In Gunnerkrigg Court, Jeanne's ghost, despite superficially resembling her living self most of the time, has a rather terrifying visage while she's in killing mode. She's been down there so long that her ghost has started to rot.
At least some vampires in Sluggy Freelance can switch between normal human and pale, pointy-eared, glow-eyed appearances. Hell Mouth and Vrykolakas vampires seem to look unusual all the time, but at least Lysinda Circle vampires have this power. In "Boys' Night Out", Torg is shown with the same kind of appearance change even though he only thinks he's become a vampire.
Whateley Universe: Like Leland Gaunt, Carmilla only looks human. She's a budding cosmic entity. In "The Turks and the Geek", she holds Shadowolf with dozens of tentacles and splits her face open down the middle, revealing something so horrific that he wets himself.
There is a meme called "Show Me Your War Face," which is quite close to this. It became deadmeme when the faces chosen were just derpy enough to miss the target for over-the-top-haminess, and people left it alone. There were some good ones, though, and they were ususally screen caps of a Game Face. Others just were.
As mentioned above, real animals do this all the time, usually by making themselves look bigger. For example, cats arch their backs and make their fur stand up, pufferfish suck up water to inflate themselves, and cobras look like any other snake until they open their telltale hoods for intimidatory purposes. Seeing a guinea pig fluff up to look intimidating is probably nature's Crowning Moment of Funny.
A more subtle version is for people to hold up their heads, puff out their chests, and hold their arms out slightly in a sort of body builder pose. There is a whole stereotype about short people who use this to try and compensate around taller people, to varying degrees of success.
An excellent juxtaposition between Game Face (predation front) and play face (social front) can be seen seen at TED during Stuart Brown's lecture on Serious Play in the interactions between a polar bear and a wolf. The scenario is at 00:01:27.
Behold the Eastern Hognose Snake, a harmless, even comical-looking serpent that puts on a heck of a show if anyone tries to eat it.
Considered to be a big reason why Armadillos are known as "Texas Speedbumps". They have poor eye-sight, so when they suddenly realize an automobile is bearing down on them, they rise up on their hind legs to be more intimidating. Which makes them just as tall as the car's bumper.
Horny toads (okay, okay, desert horned lizards) are rather known for this. When threatened, they will not only puff themselves up to appear larger, they also squirt blood out of their eyes.