In Warhammer and Warhammer 40000, 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.
GURPS alows a player to build their own version with the alternate form advantage.
Ragnorra, the obligatory Body Horror entry from Elder 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.
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.
In Exalted, there is a powerful necromantic spell named the Birth of Sanity's Sorrow. It is cast at the moment of the necromancer's death, which 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 boss-level NPCs.
The errata document for the game 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, some healing powers, and the inability to be killed unless this enhanced form can be dealt with.
The Sorcery spell Incomparable Body Arsenal turns the caster into a Humungous Mecha.
Other powers that can cover this include certain Lunar Knacks and Infernal Shintai Charms.
The Infernal 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.
The Shintai charms, explained in further detail: 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.
Averted with Asmodeus in Dungeons and 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.
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.
The Neopets' 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.