As the power of an evil creature increases, they become more of a large, disfigured abomination — until they cross the Bishonen Line, beyond which as they gain more power, they (re)gain humanoid form.
A possible In-Universe justification for this is that what's increasing is not only the creature's power, but also how much they control that power — the monstrous, oft-visceral features are the result of power pouring out without restraint. Once the creature manages to get a grip on their abilities, they can suppress the giant-scary-monster aspect of their abilities while freely accessing the kick-ass part. In some cases, this concept will be alluded to via the creature starting to revert to its monstrous form as it is harmed and loses control over itself.
On some cases, this could also be viewed as the creature being Functional Genre Savvy. For example, a huge monstrous abomination cannot physically participate in the impressive martial arts stuff that a Fighting Series is built around, so if it's in one of those, it gains an advantage by reverting to a humanoid form that will fare better Kung Fu Fighting.
This trope is also fueled by several out-of-universe factors. One, for example, is the Rule of Perception: You can only add so much horrid detail to a monstrous creature before the design becomes busy and adding extra eyes, putrid tentacles, etc. just does not make that much of a visual impact any more. The only possible way left to get the audience's attention and signal "major change" is, paradoxically, to take all the monstrous features away. Also, drawing or rendering an ever-expanding monster is taxing in terms of resources; this is a particularly important consideration in animation or sequential art, where the same thing must be drawn over and over again — so much so that in those mediums, you can probably expect the humanoid form to appear in more episodes than the monstrous form.
The Uncanny Valley has shades of this, namely how halfway along the progression to a more humanlike form, the increased anthropomorphism actually makes something seem more monstrous, after which it suddenly becomes passable for a human being.
See also Monstrosity Equals Weakness, a Super Trope of this, where more monstrous characters will be inherently weaker / less important than humanoid ones; Sculpted Physique, which may be applied to "cosmetically" or grotesquely; Monster Lord, where this trope divides leaders and mooks inside a race of monsters; and Humanoid Abomination.
Compare The Man Behind The Monsters for an exogenous demonstration of this, and Crystal Spires and Togas, which is more or less this trope applied to entire civilizations. See also Ultimate Life Form, which often does this.
Not to be confused with Bishounen Train Lines.
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Anime & Manga
In Seimaden, the ruler of the demon world is Laures, an impossibly beautiful man. Demons lower in rank than himself are attractive but more bestial. Mid-rank demons are horrible monsters. But Laures' assistant and servant Tetiyus, a low-rank demon, is almost as beautiful as his master.
From the original Dragonball manga up through the Saiyan saga, the most powerful Saiyan form is the Oozaru: a mindless giant ape.
The B-Line is first crossed with the advent of the Super Saiyan form. In this form, a Saiyan's hair turns blonde and spikier.
Super Saiyan 2 is merely a shinier version of Super Saiyan 1.
Super Saiyan 3 takes a couple steps back to the ugly side of the B-Line. The most prominent change is that a Saiyan's hair gets extremely long, but his eyebrows also vanish and his forehead grows, making him resemble a Neanderthal.
Super Saiyan 4 (from Dragon Ball GT) is reached through a different path. The first step is to reach Super Saiyan 1, then uncross the B-Line by becoming a Super Saiyan Oozaru (which is about as scary as you'd expect). Then, they have to take control of their rage in this form, at which point they cross a different B-Line; they shrink down to a primal-looking but still very human-like form that has all the raw strength of the golden ape form, while regaining their usual intelligence and personality.
Frieza's intermediate states are larger and more monstrous than his delicate-looking final form. His second form looks to a large extent like a much bigger version of his first, plus his horns become bull-like (allowing them to gore an opponent, which he does in fact do). His third form looks like an even freakier version of the Queen Alien, except in Frieza's standard two-tone purple color scheme. His final form, though, looks almost human (aside from the tail, reptilian/alien ears, and prehensile feet), and even seems downright unimpressive, even to the other characters, until he starts kicking ass. He bulks back up when powering up to 100%, though.
Cell's initial form looks like some kind of a reptile-insect hybrid. His second form is slightly more humanoid, but bulkier and even more monstrous. His ultimate form, shorter than the last one, looks a lot like a very chiseled bishounen wearing reptile/bug armor. This might be justified because Cell transforms by absorbing the Androids, who are made from and resemble normal humans.
Majin Buu's final form is that of a boy ("Kid Buu"), as opposed to the hulking Fat Buu and the muscular Super Buu. Unlike most examples, Kid Buu isn't technically his strongest form, but it's still the most dangerous. Fat Buu represented a fusion with the bulky and pacifistic Great Kai, which in turn tempered Buu's desire for destruction. Super Buu fused with a bunch of warriors, and put off wiping out humanity for as long as possible so he could get a better fight out of the heroes. Kid Buu was weaker than any of the above, but he destroyed planets with the glee of a child destroying someone's sand castle.
Super Buu: Mystic Gohan Absorbed (Buu's strongest form), in comparison to Fat Buu, plays this trope straight with a nose and genius intellect inherited from Piccolo, a more humanoid shaped face from absorbing Gohan with his signature Turtle School shirt and a much larger headtentacle.
Janemba of the post-Buu movie originally was a giant yellow marshmallow man, but became a sleek, muscled demon of human shape and size in his powered-up form.
In Dragon Ball GT, the Super Saiyan 4 transformation initially turns Goku into a giant gold ape (the same as before but in Super Saiyan mode), but when he reaches full power, he reverts to a very buff humanoid with some fur. Arguably, SS4 could be considered the Bishounen Line of Super Saiyan transformations. Super Saiyan mostly gets spikier hair, Super Saiyan 2 gets really spiky hair and bigger muscles, SS3 gets bigger muscles, huge hair, and the face tends to get less human (emotionally, but it changes physically as well), then Super Saiyan 4 goes to the same as the normal Saiyan but with red body fur and marks under the eyes. Super Saiyan 2 also has a Bishounen Mode effect with False SS2: Vegeta and Trunks get more and more buff until they're too slow and can't quite focus their power, then Gohan breaks through to true SS2, which is spiky hair and muscle tone instead of bulk.
Hollows start out as 7-10 foot monsters, get bigger until they reach the size of a skyscraper (Gillian-class, Menos level), then get smaller (past Adjuchas-class, Menos level), until they are the size of normal humans. At this point, the Vasto Lorde-class Menos — who are so powerful they are basically humans with masks — are only speculated on (a handful of characters who might be Vasto Lordes have been introduced, but it's not been confirmed), but the Shinigami are rightfully terrified of the idea of one deciding to attack.
So far we have only seen one confirmed Vasto Lorde in full: Harribel◊. Surprisingly, she never particularly liked fighting, even as a Hollow.
Aizen does all kinds of bad things to this trope. He starts off as a bishonen, evolves into a featureless humanoid, evolves into a DIFFERENT bishonen with a hint of One-Winged Angel, then goes properly OWA in the wrong order, starting with the 'angel' and finally ending in 'bizarro'. It's not so much the Bishonen Line as the Bishonen Scribble.
Inverted with Yammy, who actually gets bigger and uglier with each power level.
YuYu Hakusho has an interesting take on this trope. Most of the antagonists on the show are in a human form but the ones that are fought during the Saint Beast and Dark Tournament arc seem to follow this. During the Saint Beast arc, the four leads have to fight the Four Saint beasts and each one looks more human than the last going from Genbu (the giant talking rock), Byakko (the giant white tiger), Seiryu (the giant blue humanoid dragon) and finally their leader Suzaku the most human looking out of the four and the absolute strongest and most sadistic. Then in the Dark Tournament most of the more humaniod opponents are shown with varied levels of strength. Take the very first round against the Rokuyokai team. Roto who fought the Bishonen Kurama was the weakest and looked much less human than the stronger Rinku, Zeru and Chu who was the strongest. This is somewhat averted though with the Younger Togoro who gets less human as his power increases.
Also based on the Random Power Ranking of demons, we start from very animalistic and brutish E and D Class demons to the semi-humanoid C Class with all shorts of technicolor hair then taking a huge jump in Bishōnen with the B Classes who's only differences from humanity is their height or a hair of a different color, leading to a downgrade where the A Classes look like grotesque alien freaks, and finally ending with S Classes which could be best described as humans with the occasional horn, extra ears or different hair/skin tone.
Digimon does this quite often. In fact, a pretty good way to tell if a digimon's reached their perfect or ultimate form is by how human they look.
Palmon/Lilymon is a perfect example of the trope, going from short, cute plant-monster to giant cactus with boxing gloves to beautiful, pink humanoid.
Or just compare either Agumon line (Small Dinosaur > Big Dinosaur > Even Bigger Cybernetic Dinosaur > 7ft-ish semi-reptilian knight).
And then, of course, Omnimon.
In fact, virtually all Adventure digimon work like this. In addition to those mentioned above, Garudamon, Angewoman, Angemon and Zudomon are all more human-like than any of their lower forms.
Avoided with Tentomon, who doesn't really look remotely like a human in any of his forms, but the only real subversion in Adventure is Gabumon; he gets more wolf-like with Garurumon, then more human-like with Were Garurumon, then more robotic (and notably back to being four-legged) with Metal Garurumon.
Though also subverted in the cases of the few non-partner Digimon who can digivolve (Leomon, Etemon, Myotismon), all of whom start off relatively humanoid and then get less so.
In Adventure 02, there's a mix. The three new digimon all have one more humanoid Armor Digivolved form (Flamedramon, Shurimon and Digmon) and one less so (Raidramon, Halsemon and Submarimon). Their Champion stages look more or less the same as their Rookie ones. While the DNA forms are notably more humanoid, Imperialdramon is not, until he switches to fighter mode.
As Guilmon is explicitly based on Agumon by fanboy Takato, the above can describe his line as well. Minus the reptilian part. The part that looks like Guilmon's head is actually on the outside of his armor, and wouldn't be part of the wearer if there was a body under it.)
Zig-Zagged in Digimon Frontier, however. In this series, the characters turn into the digimon, and start out with the "Human" spirits, which are generally all humanoid-looking. However, then they attain their "Beast" spirits, which are generally more powerful, harder to control, and look more primal and animalistic. However, The Hero and The Lancer gain higher forms by combining several of the spirits together, all of which are back to being humanoid, if ever-larger. It should be noted, however, that several of the "combined" spirits created for the card game, which never appeared at all in the anime, were more animal-like than humanoid, making the results varied. One of them is a giant penguin.
More Zig-Zagging in Digimon Savers, Agumon and Gaomon's ultimate/mega forms are by far their largest, but also most humaniod.
Most series will subvert this if villains have an evolved form, with One-Winged Angel in full effect. Myotismon in Digimon Adventure has him go from a human-sized vampire to kaiju-sized demon. Similar with Lucemon in Frontier, whose final form is a giant dragon controlled by an insect-like creature.
It's actually discussed in Digimon Tamers, as the Four Holy Beasts and the Devas, wanting to differentiate themselves from their human creators as much as possible, took on giant animal forms. Vajiramon is scornful when Renamon crosses the line as Taomon, and Zhuqiaomon is disgusted by the partner Digimons' actually merging with humans).
Zhuqiaomon's really the only one who hates humans. Azulongmon wants everybody to get along, and the other two seem to mostly be minding their own business.
The Akuma from D.Gray-man start off looking like round things with cannons; by level two they could look like pretty much anything; by level 3, they resemble humanoid armored creatures. When they reach level 4, they resemble a freaky angel-like child with three halos and fairy wings. Straight out of the Uncanny Valley.
The demons of InuYasha don't transform in stages, but there is a visible progression in power; small demons, larger demons, monstrously huge demons, and then the most powerful, who can generally assume a human-like form, much as the title half-demon hero's father (as shown in the third movie), as well as his full-demon brother. Another example would be the referenced father of a monstrous-looking half-demon named Jinenji, a horse-demon, who was said to be a handsome man. Exceptions exist, but size is a general indicator of power, with the most powerful obviously able to hop the Bishonen Line at will. One character notes that while many demons are huge and horrific, the ones that take human form are often far more dangerous.
Soul Eater has Mosquito's transformations where he reverts his body to the form it was some number of centuries ago, getting progressively stronger as he goes further back: his 100-years-age form has an enormous upper body like a gorilla, his 200-years-ago form is streamlined and slightly feline, and his 400-years ago is a handsome young man (who can turn into a swarm of bats). Technically he had an even stronger form (from 800 years ago) that's even more monstrous than any of the others (including insect-like eyes) but it was quite easily defeated possibly before Mosquito even finished transforming.
Alvatore, the monstrous mobile armor piloted by Big Bad Alejandro Corner, has a "true form" of sorts in the Alvaaron, an angel-like mobile suit hidden in said armor.
That case is a subverio, Alvatore was much more powerful in terms firepower.
Gundam Virtue/Nadleeh. An extremely bulky and heavily-armored Gundam housing a smaller, but more powerful unit.
Bishojo line: When a Claymore overuses her Yoma power, she becomes an ugly monstrous inhuman drooling creature that likes to eat guts. Except more powerful ones, who start to maintain some degree of sentience (though still like to eat guts). And the strongest (such as Abyssal Ones) having rather glamorous forms, the ability to take a completely human form and being very intelligent and manipulative.
After sixteen monstrosities that get weirder with each new one, Tabris the 17th Angel is revealed to be a White-Haired Pretty Boy who also goes by the name of Kaworu Nagisa.
Of course the 18th Angel is humanity as a whole. You don't get any more humanoid than that.
Rebuild subverts this with Zeruel. It eats Eva-00, and mutates into a humanoid form that's even bigger than he was in its monstrous form.
The Bishonen Line in Chrono Crusade works similarly to the one in InuYasha — Weaker demons tend to look like Eldritch Abominations, while the strongest ones tend to look like Bishōnen with wings, horns, and other cosmetic extras.
Rob Lucci in One Piece uses his devil's fruit ability to turn into a giant leopard-man, but one of his kung-fu techniques is the ability to shrink himself down to normal proportions, making him much more agile.
Followed by Zatch Bell!, save for the fact that very few characters actually transform there. Weakest mamodo opponents encountered at the beginning mostly look like humans with cosmetic differences, then much less human-like mamodo, up to and including gigantic dragons, make their entry, yet the first two Big Bads, Zophise and Zeon, look like humans with cosmetic differences, and the final one, Clear Note, is virtually indistinguishable from a human, save from his monstrous Battle Aura. Then subverted, when he is revealed to have an One-Winged Angel uber-form after all.
In Naruto, both Naruto and Killerbee are Jinchuriki, they can transform into huge demons, but their most powerful incarnation is being human-sized with just the power of the demon surrounding them.
Now subverted with Killer Bee. Naruto in ( Kurama-Cloak is very transparent and just looks like Naruto inside of Kurama's head, while Killerbee in Gyuki-Cloak is completely solid.
Another heroic example is Sage Mode. Jiraiya's imperfect Sage Mode gives him a goatey, warts, webbed fingers and other toad-like features. The only physical changes Naruto gains from perfect Sage Mode are toad-eyes.
Superior is equal-opportunity in this regard—in fact, the most powerful and most human monsters, Sheila and Rossi, are both female. (Angelica's grandfather demonstrates that power does in fact make demons look human, rather than humanity making them powerful—he looks more human when he calls on his abilities.)
In Guyver, the Zoanoids are very monstrous looking, the Hyperzoanoids run the gamut from fairly monstrous to fairly humanoid, but the Zoalords — the masters of Chronos and the main enemies of the second half of the series, are all humanoid-sized with human faces.
In Rave Master, if a member of the dragon race, like Let, manages to pass their Dragon Trial and gain a human form, they become much more powerful.
Kuyo in Rosario + Vampirecrosses the linetwice; once onscreen, once off. He starts off in the Yokai Academy standard human form. His 'monstrous' form is a fox forged out of fire, with five tails, trying to kill the main character. He crosses the B-Line when he goes into his 'Ultimate Battle Form' and succeeds in killing the main character (for about ten seconds). Next time we see him in Season II as part of Fairy Tale, he's crossed the B-Line for his default battle form - now he looks more like his human form, just with six tails.
Avenger's foe, Korvac of The Korvac Saga. Once he was a deformed cyborg, with only one eye and a torso attached at the waist to a floating computer terminal. Upon absorbing the power from Galactus' ship, he transforms into a handsome, blond man with a perfect physique.
Peter Jackson had toyed with having Sauron appear in person at the final battle of The Lord of the Rings, in his angelic Maia form◊, but later wisely abandoned the thought. Some CGI sketches can be seen in one of the DVD bonus documentaries.
The Terminator series could be seen as an example with the titular flesh-covered robots developing according to these lines as a series. From hulking Ahnold the later Terminator models seem to be heading towards ever more slender and graceful ones, from Robert Patrick to Summer Glau. It does have some justification in the Terminators being meant for infiltration and smaller people stand out less in a crowd and the diminutive stature makes humans less wary.
In Master of the Five Magics by Lyndon Hardy, the Demons have a hierarchy of power. The least powerful are near human, but they grow larger and more bizarrely monstrous as they become more potent. The Archdemon, however, has the form of a lightly built human male. Interestingly enough, a demon's physical power has very little to do with his rank: a demon's rank is based on his ability to make others submit to his will. Each demon has a different method, some inflicting pain on others, others use pleasure, or despair, or any number of other emotions. The Archdemon, however, can use ALL of these abilities, drowning his opponent in an ocean of conflicting emotions.
The Divine Comedy: Souls become less and less human in appearance as Dante makes his tour of the afterlife, going from Hell to Heaven — from physically human in appearance in Hell and Purgatory (though the damned often have their human forms disfigured and transformed in horrific ways) to ethereal faces in the first sphere of Heaven, shining balls of light with discernable eyes in the second sphere, and beautiful but featureless balls of light in the third through ninth spheres — but when he reaches the Heavenly realm entirely beyond physical existence, the Empyrean, everyone is entirely human again.
In Lois McMaster Bujold's The Sharing Knife series, the monsters of the series are known as "malices," creatures of sentient destructive energy which drain the life force of all surrounding creatures in order to build horrific, misshapen forms, and which grow geometrically stronger the more they feed. Over the series we see several different monstrous forms of malices, such as a werewolf malice that fed on wolves and a flying malice that fed on bats. The most advanced and most dangerous of the malices they fight is one that has consumed so many human lives that it has developed a stunningly beautiful, superhuman form.
The Gruffs follow a Bishonen Line. The first ones we see — the youngest of a group of related faeries — are goatlike but about the same size as humans. The next group we see are physically similar to the first ones, but are bigger and are carrying submachine guns. The third time we see one, this one works solo, and he is even bigger than the last group; he has to bend almost in half just to fit through a door. He is large enough for his size to noticeably reduce his maneuverability. The final one is powerful enough to have defeated three high-ranking Council wizards, but he is only five feet tall and definitely more human-shaped than any of his brothers.
The Denarians play around with this trope. Nicodemus, the most powerful, is always in human form, while the second most powerful, Tessa, can become a praying mantis. The other Denarians take a variety of forms, with Rosanna and Deirdre among the most humanoid.
In the series finale of Power Rangers Jungle Fury, the restored Pai Zhua masters summon a higher level of the powers that the Rangers use. It turns them into furries. At the climax, however, the Rangers summon the highest level of power, reaching a level never seen before. This involves unmorphing and throwing CGI fireballs.
It and its counterpart Juken Sentai Gekiranger averted this with the Big Bad, however, as its true form is a giant quadrapedle-multiheaded dragon, one of the franchise's few non-humanoid monsters.
Anubis on Stargate SG-1 does this. For most of the series, his appearance is that of a cloaked figure, revealed to be a dark, shadowy form of energy. When said shadow-being loses the forcefield containing it, Anubis then takes to possessing various human hosts (seemingly "burning them out" in the process). His "final form" (at least, the last time we see him) is a creepy fat guy in a wafflehouse. note That isn't his actual form, though, it's just a projection, as nothing in that place is real. His final form would be whatever human body he was in during that episode. Same difference, for the purposes of the trope.
Asmodeus, king of the Nine Hells and most powerful Archdevil of all, is most commonly encountered looking like "an unprepossessing humanoid with red eyes, horns, and well-tailored robes." Recent sourcebooks have increased his height to thirteen feet, though. (But it is mentioned in a few books that his true form is some kind of monstrous, serpent-like beast.) This example is pretty clearly based on certain depictions of Satan. Lucifer was traditionally both the most powerful and the most beautiful of angels, at least prior to his Fall. (And, in some versions, he retains both qualities even afterward.)
D&D also has the slaadi, extradimensional spirits of Chaos which, as they get more powerful, get bigger and scarier — until you get to the Death Slaad. Although vastly more powerful than all the others and the only slaad that's naturally evil, death slaadi are human-sized and shaped. Sorta this trope without the "Bishonen" part. And then they leave the trope with the Epic Level Handbook White and Black Slaad — the white are gigantic and the black are vaguely froglike things made of pure darkness.
Many lines of extraplanar beings follow this rule. for example the Lawful Neutral Modrons: the lowest have basic geometric shapes with eyes and arms; the higher you rise in their ranks, the more humanoids they are.
Daelkyr of Eberron, the ultimate rulers of the plane of madness, lords and creators of monstrosities such as beholders and mind flayers, resemble "perfectly formed athletic human males, possessing unearthly beauty."
Shows up occasionally in the Warhammer and Warhammer 40000 universes with certain daemons and Chaos-possessed folk, mostly involving Slaanesh.
Generally proceeds like this: the weakest or stupidest daemons are entirely inhuman and animalistic. Daemonic foot-soldiers are bestial humanoids, but lack intelligence beyond basic tactics. Greater daemons are gigantic bestial humanoids that are basically avatars of their respective god and can lead entire armies of their ilk - depending on the god they might also be dangerously intelligent. Daemon princes are humans raised up to daemonic level - they're on par with greater daemons, but generally more humanoid and more dangerously intelligent. Finally in Warhammer 40,000 there are the Daemon Primarchs, who were demigods before they were raised to daemonhood and are now some of the most dangerous beings in existence. They can assume human or indeed any form seemingly at will, and will kill almost anything that gets in their way.
Daemonic possession usually inverts the line - as the daemonhost becomes more dangerous, its form becomes less and less human until it reaches One-Winged Angel status. If the host is destroyed the wrong way, though, the daemon proper might emerge crossing the Bishonen Line.
Larger Tyranid creatures become bipedal, but never cross the Bishonen Line. The Magus of a Genestealer Cult does cross the line, appearing almost entirely humanoid but being one of the most dangerous and inhuman members of the cult.
Omega from the Mega Man Zero games starts off huge in his first form, then becomes even bigger in his second, and finally reverts to a much smaller form, which just happens to be Zero's original body. As the trope states, he is infinitely more powerful in this form than he is in his others. He's actually a departure from series norm, which tend to be small, agile humanoids followed by a giant monstrous form. However, in early games, the humanoid is more dangerous than the monster, since Zero's most powerful weapons are close-range, which is harder with the boss's speed. Later final bosses show greater damage and reach on their attacks. (Compare Ragnarok-Weil to Copy X or Elpizo.)
The Big Bad of Mega Man ZX Adventsort of fulfils this trope: he starts in humanoid form, his second phase is a giant robot dragon/hydra and his last phase is angel-like. However the dragon is not his transformation, but rather his throne's.
Played straight and averted at the same time for all the dragons' Super forms in Dragon Seeds. They evolve from Baby, to Adult, and to Senior, gradually becoming more complex and massive each time, with one dragon type even becoming a harrier! From there, they can branch off into two categories: Old, where they retain their Senior form, but lose a bunch of stats and only live for a few more days (dragon years), and Super, which plays the trope straight, and they revert into a more humanoid form that looks relatively small, but becomes biologically immortal and ungodly powerful. So powerful, in fact, that you can't do anything else with the dragon but defend the title of World Dragon Champion (assuming you did get to that point) and duel against another player.
Nintendo realized in The Legend of Zelda: The Wind Waker that the humanoid Ganondorf was a far more compelling character and thus a better arch-enemy than his boarlike bestial form Ganon, so they saved the duel with Ganondorf for last, following a battle with a giant transforming puppet version of Ganon. In Twilight Princessthey did it all over again, giving the big man two human phases after you'd already dealt with his bestial form.
In Final Fantasy VII, mad scientist Hojo seems mostly normal in his weakest form, except for strange movements and a slightly greenish tint to his skin. Defeating him in this form allows you to fight Helletic Hojo, a huge and ugly mutant. When Hellectic Hojo is defeated, he becomes Lifeform Hojo N/A, an eerily-beautiful humanlike creature draped in white.
Final Fantasy VII: Crisis Core does this too, with Genesis Avatar, a hundred-foot-tall Evangelion-esque monster knight, as the first form, and the final battle with Genesis being a straight sword duel after he's inexplicably reverted to his normal and non-decayed form. Gameplay-wise this may or may not be an example. The Avatar is going to be a long and tedious, if not difficult, fight regardless of what you do. The human form can and will mess you up provided you go into melee. It can be stunlocked by magic, making the fight something of a joke.
The final boss of Kingdom Hearts II first fights in his original black-cloaked form, then changes into an armored knight on a throne which is itself on the head of a robot dragon; after several fights against said dragon and knight, the final fight is against him in a recolored version of his original outfit, with an upgraded version of his original fighting style.
While Heartless, as a rule, grow more massive and horrifying the stronger they are, the most powerful of them all, Ansem/Xehanort's Heartless, is visually indistinguishable from a White-Haired Pretty Boy normal human, though he's the exception to the rule. However, while the usual Nobody Mooks of the PS2 sequel are somewhat-inhuman beasts, with a few massive Nobody bosses, the frighteningly powerful Nobodies that make up Organization XIII are visually indistinguishable from normal human beings.
Before Xemnas however, was Marluxia. The first fight is straight up against him, using his own flower magic and wielding a Sinister Scythe. The second has him on a sinister, large, floating mech with scythes for arms. The third fight takes place on top of the remains of said mech, only this time Marluxia has a creepily beautiful angel-like familiar wielding a gigantic scythe backing him up and basically doing all the fighting for him.
In Kingdom Hearts Coded, the digitized Sora's Heartless first takes the form of a Darkside with glowing tattoo-like markings, but after taking a beating transforms into a dark copy of Sora, then finally three dark copies of Sora.
Ryu, the protagonist of Breath Of Fire III, can transform into various dragon forms. His strongest form, Kaiser, is identical to his human form, except recolored gold. When you use that form's Kaiser Breath attack, however, he temporarily changes further into a gigantic dragon. This example is more justified than most, however, because the Kaiser form is just so frickin' huge that having Ryu fight permanently in it would fill up half the screen.
In Brave Fencer Musashi, the Wizard of Darkness is originally a massive, horned demon-man; he changes into an even more monstrous green beast, then finally into a white-skinned humanoid slightly larger than Musashi.
Eve's final form in Parasite Eve is a rare Bishoujo Line example, having gone (over the entire game's course) from normal actress to 8-foot pale seductress with a serpentine lower body to taking up a good-sized room while preggers with the Ultimate Being to 20-foot floating monstrosity covered in grasping arms to functionally nakedFragile Speedster harpy-thing.
The characters of Bloody Roar tend to avoid this trope, since turning into animals is the whole point, but a few characters get past that point.
Cronos, one of the potential final fighters, turns into a very nonhuman penguin when he powers up a little. Turn on the Ominous Latin Chanting, however, and he can transform into a phoenix that looks like a human with a beak and feathers before he kills you.
Uranus is an even better example, as she first turns into a fearsome chimeric beast with her normal powerup, but her strongest form looks like her normal one, just glowing.
Uriko also exhibits a degree of this. In the original game, as the Final Boss, her beast transformation consisted of going from a little girl to a full-grown woman, sprouting cat ears, and her hair turning green. Later, when she's less powerful, her beast form, although still less animalistic than the others, is noticeably more feline. She also exhibits going to the One-Winged Angel again, as in the first game, her ultimate form is a grotesque chimera.
The original game features this. Although Yuri's fusion souls are all roughly human-sized, they generally get more bizarre or monstrous as they get more powerful, with the second-most-powerful, Amon, being a hulking, monstrous demon with a blade on one arm. However, his most powerful, the Seraphic Radiance, is just him, glowing white, with black wings and mystic-looking tattoos.
Even more so in Shadow Hearts: Covenant. Yuri's low level fusion forms look like 'elemental' costume swaps of his normal body. His stronger forms include a giant armored knight, a beastly rock monster, and a sea dragon, all much larger than a human. Amon, as usual, is a big, muscular demon, and Dark Seraph looks like a black-winged, almost completely naked, but otherwise fairly normal Yuri.
In the original, prior to the third fight with Nelo Angelo, he reveals his true face ( identical to Dante's, as he's his twin brother, Vergil). He still has blue skin and glowing eyes, but he's more human than before, and has some new, infuriating attacks.
If you count the Despair Embodied of Devil May Cry 2 as a second form of the immediately preceding boss, as some Fan Fic writers do (it's not made clear in canon), this would apply. This does not occur in the other games of the series.
In Viewtiful Joe, it is revealed that Captain Blue, Joe's movie hero, is in fact the true villain of the piece. He transforms into his monstrous "King Blue" form that is so large, Joe must face him in his Humongous Mecha "Six Majin". After the battle, Joe jumps to the nearby platform to commence the final battle; Blue appears again, much younger and more muscular than he ever was before. After being defeated, he reverts back to his older, overweight form.
Dhaos from Tales of Phantasia fights you as is, then turns into his presumably true alien form. After beating that, though, he returns to being a humanoid, albeit very tall, suited in white, and sporting angel wings.
Granted, that form was only put into the Remakes.
The final boss of Tales Of Hearts fuses with the core of a giant planet-eating parasite to fight you, has a big magical lump of flesh for an ally, and can summon monsters, walls, and spikes from the room itself to attack you with. After beating that, you enter his mind and fight his "true self", which is him with a bunch of power crystals, a silly collar and powerful spells, a tough defense, and a Limit Break.
The final boss fight is against Malpercio, whose body is a hideous frankenstein made up of the body parts of various dead gods. After he's beaten, though, Melodia combines with him, turning him into a more humanoid figure with wings on his head.
In Treasure Of The Rudra, Mitra fits the theme, as she starts as a statue-like being, then a giant half-human, half-serpent monster, and then assumes a smaller, more humanoid form for the final battle.
In Symphony of the Night and Harmony of Despair, Dracula's final form is a massive dragon, but Dracula's human form is visible slouching in the center of it.
Graham in Castlevania: Aria of Sorrow has a One-Winged Angel transformation, with him sitting comfortably within what appears to be two giant pale women joined at the eyes and ribs. Dracula in general, with Dracula's incaration here as Soma Cruz, later in the series, being by far the bishiest.
Lampshaded in Castlevania Lords Of Shadow, where the bestiary mentions that vampires become more human-like as they become more powerful.
At the end of Metroid Prime, the title character sheds its giant black arachnoid armor, revealing a mixture of an octopus, a Metroid, and a human face. While practically easier than its previous form, it cannot be hurt by weaponry not based on the substance it feeds on and produces (Phazon). Samus did have the ability to use that material, but anyone else would have been screwed.
The trope is played even straighter with what happens after you defeat Metroid Prime's second form though: it turns into a Phazon clone of Samus' armor.
Also, at the end in Metroid Prime 2, the Emperor Ing goes from being what resembles a giant eye, to gas spewing shell, to a much larger version of the basic Warrior Ing, which isn't humanoid at all, but would be this trope from an Ing's point of view.
In the episodic computer game Time Runners, the main character (a teenager from Earth) travels through time and dimensions to fight monstruous "Chronodemons": a robot, a cyborg, a sentient spider, a necromancer, a mutated human and a zombie pharaoh. For the final duel, is pitted against the creator of the Chronodemons: an exact lookalike of him.
Metal Gear Solid 4: In the boss fight against Ocelot, he slowly regresses in his mind to his Bishōnenyounger days. However, since this is only in his mind, he remains looking like an old man, and only his movements become more 'bishounen'. Note also that the Life Bars change as well!
The final boss of Castle Crashers is an evil wizard who first fights you by controlling crystals on a throne, then fights you on foot with a magic wand, then bloats up like a balloon and drops magical bombs, and finally becomes an enormous bat creature. When you defeat that, he regresses back through his balloon form to his normal form, where he summons a gigantic sword for one final battle.
In Zone Of The Enders there are gigantic battleship-sized Orbital Frames, but the most powerful Orbital Frames ever made are comparatively small and bipedal. They get more powerful when they lose a lot of their armor and look even more human-like.
Wesker in the Resident Evil series, in a rare out-of-boss-fight example. Despite being essentially a man in a trenchcoat with red eyes, he's more powerful than all the horrific genetically-engineered monsters in the series. He seals his fate by going One-Winged Angel for the Final Boss fight of RE5, which allows you to (presumably) finally kill him for good.
The Dark Force appears multiple times. First, as a monstrous head and shoulders attached to the satellite, and second in the form of a giant spider with the same head. Then it hides inside the form of a human archaeologist with dark magical powers, allowing it to hunt for the Aeroprism. When the party finds the Aeroprism, he transforms into the third Dark Force, which is a humanoid demon much smaller than either of the others.
Even more the case with the Profound Darkness. First form: a collection of mouths and red eyes on a blob of purple flesh. Second form, it's got green-gray armor, plenty of appendages, but only one mouth and not as many eyes. The third form? A giant gray human female with bony wings and seven eyes in symmetrical positions on the thighs, wings and forehead; if you hadn't seen the prior forms, you'd probably take them as gems.
The Elder Scrolls III Morrowind: Big Bad Dagoth Ur spreads his influence via the corprus disease, which drives its victims insane and causes horrible tumorous growths. For most this means becoming a grotesquely deformed zombie, but the higher echelons of the hierarchy are able to control these transformations, to the point where Dagoth Ur and his immediate underlings look completely humanoid except for the occasional extra eye (or pair of nipples).
Inverted in Oblivion with vampires. The longer a vampire goes without feeding the stronger they become in vampire abilities, and thus the more vampiric they look, eventually causing townspeople to freak out and guards to attack.
Hunters from Prototype are massive, ugly hulks. Leader Hunters are even larger, even uglier and even more powerful. The Supreme Hunter, on the other hand, while impossible to call handsome, is still slightly better-looking than the previous two, not that much bigger than Alex, and predictably even more powerful than Leader Hunters. Alex himself is also bordering on physical godhood by endgame, though he usually looks like a normal human. PARIAH looks completely human, and as far as the Web of Intrigue can say, is capable of single-handedly wiping out all life on Earth.
Many Pokémon get more humanoid as they evolve; Abra to Alakazam, Torchic to Blaziken, Mareep to Ampharos, just to name a few examples. Some subvert this, such as Togepi.
In Spectrobes the Krawl start off as relatively small monsters, growing in size slowly with each passing boss until the final boss who is relatively large, basically the largest enemy of all (to say nothing of your ally Tindera). Immediately in the second game you are introduced to the high Krawl who are human shaped Krawl. but then this trope is subverted as the high Krawl all go One-Winged Angel. but then this is Doubly Subverted with the final boss(s) who are all massive and only grow larger until you face the ultimate Spectrobe, whom you cant defeat. then this trope is somethinged by the True Final Boss being Krux, a human.
In The World Ends With You, when Sho Minamimoto becomes part Taboo Noise, he basically just loses his Nice Hat and gains another black hand; much more human than his original noise form, and he becomes, in his own words, strong enough to rival the Conductor.
Street Fighter II: the Sonic Boom user was Guile who has a very angular face, chiseled jaw, and huge flattop. Then Street Fighter Alpha came and Guile was replaced with Charlie (except for SFA3 Console) who wears glasses and has a more handsome face. Then Street Fighter III gave us the bishonen Remy. IV subverted it by swinging back in the timeline to just after II, returning to Guile.
The Big Bad of Noitu Love 2. For phase one of the boss fight, she's a blobby sphere with legs. For phase two, she transforms into a bat-winged human, and grows so big that only her head and shoulders fit on-screen. For phase three, she loses the wings and shrinks down to only slightly larger than the protagonist.
Mirelurks evolve like this in Fallout 3. Standard ones are basically giant bipedal crabs, then there are Hunters, Nukalurks, and Swamplurks (Point Lookout), with extra limbs and spikes on their shells, and this culminates in Mirelurk Kings, which look like mutated humans and attack with Psychic Powers rather than tackle attacks. Bonus points for them resembling Frieza and Cell. Mirelurk Kings and Swamplurk Queens are actually a separate species than other Mirelurks, being mutated versions of turtles, making their differing appearance justified in a weird way.
In the main game, Super Mutant Behemoths are 20-foot tall Dumb Muscle brutes with Scary Impractical Armor who only wield fire hydrant hammers as weapons. Broken Steel adds Overlords, which are closer in size and intelligence to the standard Mutants, but with HP near that of a Behemoth and much deadlier weapons such the Tri-Beam Rifle and Gatling Laser.
Around early July, Teddie, after recovering from his ordeal with Shadow Rise and Shadow Teddie and poking into the real world, he talks about how he's grown on the inside. And he has, literally: when he takes off his bear costume, we see not empty space, but a blonde, blue-eyed bishounen.
Inverted with with Izanami, who starts the fight as a woman in a strait jacket and a massive skirt, before revealing her true form, a monstrous, multi-limbed, rotting...thing with what's left of Izanami's upper body from the last form on top...which is also rotting and unpleasant to look at.
Ramirez in Skies Of Arcadia does this when after going through his One-Winged Angel phase, by fusing with the gigas Zelos, he crosses the Bishonen Line once his second form is defeated and resembles something of a silver angel with his original human form clearly visible.
The first time you fight Dragon Quest VII's Big Bad, the demon lord Orgodemir, he first starts out as a humanoid creature (with pointy ears and wings), but halfway through the fight, he goes One-Winged Angel and becomes a monstrous demon centipede with new attacks and higher stats to use on you. When you fight him at the end of the game, though, he starts out as a palette swapped version of the centipede, and halfway through the battle, he crosses the bishonen line, becoming a palette swapped version of his original form with much more powerful attacks and stats. He then further changes into a hybrid of the two forms before changing into his final form, which is just loaded with Squick.
The final boss of Deadly Premonition first mutates into a bloated version of himself, then into a giant wall-crawling lizard-thing with backwards hands, and finally into a giant version of his normal self. Subverted from the norm for this trope because his normal form is a goofy-looking fat man in overalls.
The Big Bad of Lunar Eternal Blue, Zophar, takes a giant Eldritch Abomination form when you first seen him. When you fight him for the first time, he looks decidedly...prettier, even when he goes One-Winged Angel on you. Both forms are also nowhere near the size of his original form, which he actually turns into the game' final dungeon.
In Chrono Trigger, Lavos actually subverts the Bishonen Line. His colossal bug-like exterior is actually a protective shell guarding his true humanoid form, which, probably thanks to Akira Toriyama, happens to look a lot like Perfect Cell (see the trope picture above). However, upon defeating this form, the Lavos Core reveals itself to be a shambling monstrosity, the end result of Lavos' attempts to achieve biological singularity. And, in actuality, the core is a tiny pod floating beside said monstrosity.
Asura's Wrath has several examples, being a game that lives and breathes anime tropes:
Asura has both the "Mantra" form that forgoes the previous six-armed forms to combine the power of 1000 arms into two massive gauntlets. Then there is his final "Asura the Destructor" form that has him either larger than a planet, or simply applying the "Mantra" design to all six arms.
Played with Vlitra Core. While being a humanoid boss and the most powerful form, it actually exists at the same time as Vlitra on the world's Surface, who was treated as a One-Winged Angel.
Chakravartin, the ultimate villain of the series, has multiple ostentatiously divine, many armed forms, but finally becomes "Chakravartin the Creator", a simple, skeletal humanoid wrapped in silver armor.
Reversed in Half Minute Hero's Evil Lord 30, in which the Evil Lord increases his magic power by "getting sexier", for which he must go to a beauty spa.
In Scott Pilgrim Vs The World, the final boss actually has three forms. The first of which is Super Gideon Graves, which is Gideon grown to twice his normal size, the second of which is Gigadeon Graves, a giant mutated monstrosity fought within Gideon's own subspace and the last of which is Gideon Graves, fighting unaugmented with only his pixel katana.
During the final battle in The Halloween Hack, The Id uses its mindpower to turn into a PSI-wielding, curse word-spouting superhuman version of Dr. Andonuts.
A stunningly common trend in Bullet Hell games is for bosses to become larger and/or more complex as one progresses through the game, but instead of being even scarier, True Final Bosses are small and elegant-looking. This shows up in Blue Wish Resurrection, XOP, all the Trigonometry Wars games, and probably dozens of other places.
Adventurers! is the Trope Namer, as seen above. Khrima goes from a human sorcerer, through two progressively monstrous forms, and then takes on his final form, which is far more human.
This is seen with the prototyping of the Black Queen's Ring in the Kid's Sburb session. The features of whatever's prototyped by the kids are incorporated into the carapace that wears the ring, in this case Jack Noir. The first three add a harlequin hat, tentacles, wings, cat features and a sword through the chest. But when Bec is prototyped last, the tentacles disappear,note they're later revealed to appear on demand and the head elongates from a creepy harlequin/cat mess into a sleek, canine face, leaving the carapace much smoother and cleaner-looking than before. The eye scar, wings and missing arm stay, though.
Given that a First guardian wasn't prototyped, this is averted in the Trolls' session, where 12 things get prototyped, including an Eldritch Abomination , creating a monstrous, hideous Black King as their session's Final Boss.
Lord English averts this trope as well. As Caliborn, he starts out a small, rather Creepy Cute skull monster boy. As Lord English, he is a giant, hulking monstrosity, with bulging muscles, flashing billiard eyes and a hideous skull face.
Spacetrawler: After Yuri becomes a cyborg, she starts constantly upgrading herself—her cyborg implants become increasingly bulky and inhuman, until she eventually looks like some kind of robot spider. Martina forcibly downgrades Yuri to her baseline cyborg form. Then Yuri upgrades herself to her most powerful form yet, a a self-styled death goddess—and she looks like her human self with a different skin color.
In the fourth season of Justice League, Lex Luthor is possessed by the intelligent supercomputer Brainiac, who turns his body into a hulking, half-machine, betentacled monstrosity with a robotic "face" on its abdomen, containers sticking out of his back, and tentacles coming out of his arms. Then they "merge completely" into one being with the power to remake matter with a thought, and become green-skinned Lex Luthor wearing gold armor and a Brainiac tattoo on his forehead. This "complete merger" gets broken off and then phased out by The Flash, leaving Luthor completely human again and naked.
Shows up in Sponge Bob Square Pants, of all places. In "The Two Faces of Squidward", Squidward receives plastic surgery◊ and after trying (and failing) to undo it, he comes out like this.◊
In Ben 10 Ultimate Alien, Ben 10,000's most powerful form just happens to be himself, not having any alien features (aside from the Omnitrix emblem on his chest). In this form (called "Ultimate Ben"), he can use the powers of all the aliens in the Ultimatrix without having to transform into them.
In Wakfu, Rushu at first looks huge, demonic-looking, and brimming with fire. His true form is a much smaller cyclopean humanoid demon that is actually even more powerful than his giant form. The only opponent strong enough to force him to fight in this form is Goultard, the demigod son of Iop himself.