Warriors of Chaos can be blessed or cursed with "gifts" from their patron Gods, turning them steadily more inhuman. Eventually, at a certain point, the weak-willed become mindless Chaos Spawn, while those with Villainous Willpower ascend into Daemon Princes who retain their sentience and control over their vast powers. In either case the warrior now becomes a full Daemon extra vulnerable to anti-Daemon abilities and armament.
In previous editions of 40K, Greater Daemons aligned with Chaos Space Marines do not appear with the rest of the army, but must be summoned into combat. This involves the sacrifice of a champion. This actually allows you to initiate a One-Winged Angel transformation mid-game, as the Daemon will always be considerably more powerful than whatever hapless shlep it used to be.
Nyarlathotep tends to do this in mythos RPGs. Destroying his human form often causes him to manifest as some kinds of sanity-blasting monstrosity.
GURPS alows a player to build their own version with the alternate form advantage.
Ragnorra, the obligatory Body Horror entry from D&D'sElder Evils, transforms from a bloated blob of wormlike flesh into a curiously human face made out of strands - her True Mother form. While spawning utterly abhorrent monstrosities that simply should not be.
Averted with Asmodeus in Dungeons & Dragons: While his true form is apparently a giant primordial serpent of some sort, the rules given for fighting him - as unlikely a situation it would be to fight the super-intelligent, scheming ruler of all Hell - only describe fighting him in his humanoid devil form.
Presumably because, against a being to whom the word "god" would be an insult, your party wouldn't stand a chance.
Erwin Stahler from Mutant Chronicles starts out as a human-sized model on the battlefield, but once he's down, he goes One-Winged Angel and turns into a bigger, armored mutant with big honking claws.
Lunar Deadly Beastman Transformation, or Warform, allows them to transform into a nine-feet tall monstrous merge of their animal and human forms. It comes with increase to physical parameters, host of mutations to represent the alterations and ability to instantly activate a wide array of complementary Charms to buff themselves up higher. And even if you can deal with all that and beat it down to a point when every other Exalt would lie on the ground and helplessly bleed out to death? The Lunar does not bleed. The Lunar regenerates and will get up for a round two if you turn your back on the body.
Several Lunar Knacks serve to enhance their forms further, letting them grow their animal form or warform to Kaiju comparable sizes and go a-stomping.
Infernals have Shintai charms which let them change into a monstrous form based on the Yozi the power is derived from. The most minor ones for physical changes turn you into a version of you crafted from red mist (Scarlet Rapture) or sand (Soul-Sand Devil), can split off a mortal clone of you (Splintered Gale), or allow you to copy someone else (Black Mirror). More extreme ones give you a constellation of crystalline orbs (Heuristic Logos). The most one-winged of them are Tenebrous Apotheosis (which ramps you up to become the Ebon Dragon), All-Devouring Depths (which turns you into a shoggoth), Demon Emperor (which turns you into a walking Ground Zero), Greater Shintai of the Endless Desert (which turns you into a living geographical feature) and Devil-Tyrant Avatar (which allows you to string together mutations to build your own form). Of these, only Soul-Sand Devil and Splintered Gale can't be used as a rude surprise in combat, and that's only because one is permanently in effect and the other is used to create mortals, although one odd Splintered Gale build theoretically allows you to mass-produce cloned suicide bombers. In the upcoming third edition of the game, the system gets revamped to allow you mix and match the pieces (which you get by buying other Charms) to create your own Shintai.
The Shintai Charms are augmented by the Charm Driven Beyond Death. When an Infernal hits the Incapacitated Health Level, they make like they're downed (like, say, going down on one knee). Then their anima banner erupts, and they have one turn where they're completely untouchable but can only activate a Shintai Charm. At that point, they reenter battle with their Essence pool somewhat refreshed... and at the end of the battle, they have to roll not to keel over regardless.
Even Sidereals get into this: they turn their Familiars or mortal acquaintances into massive Elemental Dragons which they can ride. Unfortunately, this destroys the personality of the pet and reduces them to a vegetative state when it's over unless the Sidereal sacrifices a dot of permanent Willpower to let them live on in their Dragon form.
The errata document for 2e includes an upgrade to the stats of the Unconquered Sun as he appears in the Glories of the Most High supplement that gives him an enhanced form called Magnanimous Unbound Sun that gives him an extra couple hundred health levels, an extra 28 arms, constant regeneration, and the inability to be killed unless this enhanced form can be dealt with.
There are Sorcery spells available to all Exalt types: Incomparable Body Arsenal turns the caster into a mechanical creature, sprouting weapons out of anywhere, while Invulnerable Skin of Bronze can complement that with increased defences.
On the high end, there is a powerful necromantic spell, Birth of Sanity's Sorrow. It activates at the moment of the necromancer's death, and transforms their dying corpse into a titanic monstrosity of grotesque design, inspired by the necromancer's delusions and nightmares of dead principles. Fortunately, BoSS is usually only accessible to very high level Abyssals and their Deathlord masters.
On the Martial Arts front, the capstone Charm of Dreaming Pearl Courtesan Style turns you into a kind of gazelle/carp hybrid abomination made out of dreams (or nightmares). This transformation can last indefinitely, but is addictive and hard to turn off, while abuse may lead to you getting dissolved into the world of dreams.
On the Blossom of Perfected Lotus, the Sidereal Martial Arts (it is noted that the previous entry is touching on that level), a couple of Styles are worthy of note here. From Prismatic Arrangement of Creation Style, Four Magical Materials Form turns you into a living Artefact with huge boosts all around (and even more if you use the Prismatic Arrangement of Creation Form capstone to combine it with other MA forms). Meanwhile, Beauty in the Eye, from Border of Kaleidoscopic Logic Style, has you vanish for a turn and return with an explosion, transformed into a titanic figure modelled after the deity that granted your Exaltation and shaped from elemental substance most prevalent in the region.
Shards of the Exalted Dream has Burn Legend, which reimagines Exalted as a kind of Tekken kung fu brawl setting. The Overdrive for Yamajin, known as Demon Emperor Shintai but bearing no similarities with the one described above, causes you to take on a monstrous form and get +3 to all your stats until you lose a health stock. Generally speaking, if a Yamajin gets up to Demon Emperor Shintai, the rest of the fight will not go well for the other player unless they get incredibly lucky or can predict what attack the big guy is about to use.
Demon The Fallen gives the titular Fallen "apocalyptic forms," echoes of their glory from when they served Heaven. Each form is based off of a primal incarnation of the Lores the angel knows; therefore, a Devil in the mold of Lucifer could have an apocalyptic form that's either a pillar of fire or a glorious, shining light, and a Devourer could have an apocalyptic form that's either a man with the head of a lion or the perfection of human flesh made manifest. But if the demon's weighed down by Torment, well... expect the apocalyptic form to look honked up.
Vampires have access to some nasty disciplines that let them do this, such as Zulo Form of Vicissitude, the armor of living shadow of Obtenebration, the War Form of Protean and Serpentis.
The Unchained of Demon: The Descent, the New World of Darkness's analogues to the Fallen, can change into their true demonic forms, which tend towards biomechanical horror, befitting their status as former servants of the God-Machine. It's also possible for a demon in dire circumstances to "go loud", assuming their demonic form at full power with a healthy store of Aether; however, it irrevocably rips their mortal identity apart, meaning they're now a big glowing beacon for all their divine enemies.
The nWOD's Werewolves have access to a nightmare warform.
So do all the werecreatures of Werewolf: The Apocalypse, mostly a huge anthropomorphic animal creature like a three-metre tall wolf-man. And then there's the Mokole, the lizard keepers of Gaia's memory. What's the warform of a race of crocodiles, gila monsters and assorted reptiles? Why, your very own dragon/dinosaur/kaiju-lookalike, individualized for every character by a list of traits including "razor claws", "poison sacs", "huge size" or "wings". They dream up their form from ages past - and some of those dreams can be pretty weird indeed.
The Neo Pets' web-based RPG parody/thingy Neo Quest II has the final boss "defeated", then returning with a few additional immunities, double the HP, and a double-sized graphic. And additional limbs, wings, spikes, and menace.