Monstrosity Equals Weakness

A trope in which, the less human or normal looking an enemy or monster is, the weaker or less effective that enemy will be, whereas less monstrous foes will be deadlier and more powerful. If all foes are humans, then you can see this in body type instead: fatter or bigger fighters will be less powerful than thinner characters.

This is mostly due to the fact that writers seem to think viewers associate monstrous figures with being strong but mindless and stupid, and thus use less monstrous forms for more effective villains. Even if that villain has a monstrous final form, he will usually be seen in his humanoid form for most of the time, his other form essentially existing only as an excuse to allow them to die. This may also be because humanoid villains become characters with distinct personalities, while the more monstrous ones are less versatile outside a fight. Thus, they tend to be Mooks, Monster of the Week or the One-Winged Angel form of a villain who appears humanoid for most of the series. Often justified in-story by the notion that the inhuman beings prefer to seem human for whatever reason, and that the more powerful or skilled ones can do so more successfully.

Humanoid in this case being two legs, two arms (though arms are one of the few things that some can get away with), one head and relatively normal proportions. Also, a relatively "normal" face as well.

Compare What Measure Is a Non-Human? and Humanity Is Superior. See also Bishonen Line, a Sub-Trope of this, where a single character eventually becomes more human-like as he gains power, or Clipped-Wing Angel, the exact opposite. This is usually the flaw in a Shapeshifter Mashup. It also occasionally overlaps with The Worf Effect, with enormous, monstrous enemies existing mostly to be cut down, to show that not even such amazing brute strength is any match for whoever is fighting it. There is considerable overlap with Monster Lord.

Media in General

 * Depending on what mythos the chosen medium follows, Vampires are usually more batlike at a weaker age, as they get stronger, they get more human-like.

Anime and Manga

 * Extremely blatant in Yu Yu Hakusho, where all the most powerful opponents are the most human ones, even if all but one of these were demons. Inversely, if a character doesn't look at all human, that's a cue that he'll be beaten in two seconds. As an almost inverse, some of the characters have demonic forms... which consist of nothing but a tail, or, in one case, cool looking eyes all over their body, but a no less human-esque than their normal forms.
 * The best example is the Four Saint Beasts, which are described by Kurama as not looking at all human. In a complete inverse of what Kurama describes, the enemies get more and more human looking as the heroes go through their lair. The weakest is a gigantic golem, and the strongest is a normal-looking guy whose bangs double as antennae.
 * The other two beasts avert it, though. The second-least-human, Byakko, is ridiculously hard to kill, while the second-most-human, Seiryu, goes down fairly easy. Considering who fought who, it may just be due to their opponents being of differing strength, but the trope is still avoided from a narrative standpoint.
 * Yes! Pretty Cure 5 plays Monstrosity Equals Weakness straight: The Monster of the Week is, well, monstrous, while the Quirky Miniboss Squad is humanoid, and the first Big Bad has only human-looking forms. The series also inverts Bishonen Line: the aforementioned Quirky Miniboss Squad tend to start out human-looking, switch to less-attractive, animal-inspired forms for battle, and have full-fledged giant hideous monster forms when it's time for the heroines to kill them off.
 * At the start of Bleach Hollows are pretty much non-human looking and a single one can be beaten easily. Once upgraded them to Arrancar, which are basically humanoid hollows, the real trouble started for the soul reapers.
 * Menos do this on their own. The massive Gillian are the weakest, and look like sky-scraper versions of normal hollows. Adjucas are significantly stronger, smaller and usually animal or plant like in their appearance. Vasto Lordes are strong enough to beat Captains, and they're human-sized and supposedly look human as well.
 * Sango, the trained Youkai hunter of Inuyasha, was taught that the most dangerous youkai are those which look human, a rule of thumb which proves almost universally true throughout the series. Of particular note is the fact that the worst defeat suffered by Sesshoumaru - who is presented as probably the most powerful youkai in the series - occurs the one time he transforms from his humanoid form into that of a giant dog, even though said transformation is presented as a power-up.
 * Actually played with in this series, as the most powerful demons are capable of attaining a human form and possess some sense of intelligence, using their monstrous form only when fighting seriously.
 * Done deliberately in Hunter X Hunter, where the freakish-looking big guy with a steel-plated fist turned out to be a terrible fighter who had some plastic surgery screwed up, hurts his hand when he punches things, and can only win fights by intimidating the other guy into giving up.
 * More typically used in the Yorknew City arc, when it came to the Phantom Troop (people with little strange about their appearance but their outfits) fighting the Shadow Beasts (weird monster people, except one guy who was just fat and ugly): when one member of the Phantom Troop fought 4 Shadow Beasts, he killed all four though he was only able to recover from his injuries thanks to his allies assistance. When the other six Shadow Beast attacked five Phantom Troop members, they were able to non-fatally incapacitate one but got killed by the other four without beating a single one of them.
 * Busou Renkin justifies this: Homunculi can only attack in the manner of the base creature. Gorilla-based can just use pure strength, an eagle-based one can use their talons, but only a humanoid homunculus can use real weapons. This includes the titular superweapons, by the way.

Comic Books

 * Averted with Fat Cobra from Immortal Iron Fist. He looks like a big fat brutal sumo wrestler. He turns out to be extraordinarily fast, strong, skilled and intelligent even relative to the superhuman billionaire hero.

Film

 * The original Final Battle of Blade involved Deacon Frost transforming into a giant Blob Monster made of blood, rather than the Sword Fight with the eponymous protagonist. The original scene didn't play well with test audiences, who tuned out and lost emotional investment in the story because of this trope.

Literature

 * Making this Older Than Television and most likely the Ur Example, Bram Stoker's Dracula, while never exactly weak, was an unnatural and horrifying old man in the first act when all he had to feed from were slaves and transylvanian commoners that were in low supply. After gorging on the blood of the crew of the ship he stowed away on, and several weeks of terrorizing londoners, he became stunningly attractive and seductive.

Video Games

 * It's a general rule in the "spectacle fighter" genre that the human-sized bosses are always tougher than the giant monstrous ones.
 * In Kingdom Hearts II, the regular/weakest of the Nobodies are twisted abominations that look like mangled, bleached clothing. Even the Twilight Thorn Boss was a Breather Boss, being easily beaten through Reaction Commands. However, Special Nobodies, like Organization XIII, have no consistently distinguishing features from other human characters, and are among the hardest bosses in the game. The Heartless, however, except Ansem, do the opposite.
 * In the game, this was explained. The Nobody is the physical form of a person left behind after their heart has been taken and turned into a Heartless. Most Nobodys literally become faceless, indistinguishable nobodies, but the strongest beings retain their original appearance (or a form close to it), basically becoming the soulless husks of former people.
 * The order of Kain's Lieutenants ascends in order of weakest to strongest. You begin with Melchiah, who is basically nothing more than a stitched together bundle of rotting flesh, and end with Kain, who is still largely humanoid despite his evolution.
 * Often used in House of the Dead in terms of its bosses, where the various bosses one fights may be anything from mutant animals to hulking giants to living plants, but the final boss will usually be a very humanoid creature that only has a bit of zombie-ness about him. This is not often used with the normal enemies, though, where the most dangerous are usually the most inhuman.
 * A good way to spot the Warmup Boss in any given game in the Mega Man X series is to find the biggest one. Chances are it's the weakest. Also applies to the intro boss, which will almost certain be a hulking wimp. Its Sequel Series, Mega Man Zero and Mega Man ZX, also use this trope, though in Zero 1 you really had no way of telling which was which till you went through the stage. The various minor bosses you face are mostly animal-form. Major villains--Sigma, Prometheus and Pandora, and so on--are humanoid. But the various bosses with multiple forms tend to become less humanoid in each.
 * Except Omega and, possibly the 2 best final bosses in the series.
 * Used straight with Haseo's B-St form in .hack GU. While it is infinitely more powerful than his second form (as in, being able to use an attack that makes Gate of Babylon look minuscule), it also hampers with his cognitive ability so much so that he gets his ass handed to him by really easily.
 * While the demon forms of Elvis, Shannon, and Belze are nothing to sneeze at, it's Azel and the Bonus Boss Evil Gene who are the most difficult bosses in God Hand, mostly due to their speed and ability to dodge your attacks, even while you're using the God Hand.
 * In the Tales (series), the toughest bosses tend to be the human or humanoid ones, due to being small targets, being able to pull off combos like the player, and often having a Mystic Arte/Hi-Ougi; privileges non-human bosses rarely have.
 * Neverwinter Nights has this one down pat. Humanoid enemies are crippled by the game engine to make them a fair fight. A level 20 fighter (assuming level-equivalent equipment) will win against almost any monstrous opponent in the game engine; a level 20 sorcerer has trouble being threatened by anything the game can keep track of.
 * Averted in tabletop Dungeons and Dragons, however. High-end enemies run the whole gamut from colossal dragons to Demon Lords and Archdevils to Eldritch Abominations to Kaiju to Physical Gods. None save the latter tend towards humans in size or looks.
 * The Draenei in World of Warcraft have two derivative species: the appropriately named Broken, and the Lost. Draenei exposed to the fel or demonic magic of the Burning Legion are corrupted and lose contact with their source of power, the Light. This manifests as physical deformations and a descent into madness. The Broken are more-or-less sane, but they have crippled bodies and are in constant physical and emotional pain. The Lost are completely insane, and even more crippled than the Broken. An analogy: Draenei are healthy trees, Broken are burned but still standing trees, and the Lost are stumps.
 * High elves, blood elves, and Wretched are arguably a similar case.
 * Averted in Dragon Age Origins. Non-boss humans and humanlike Darkspawn (essentially Tolkien Orcs) are some of the simpler enemies. Bizarre creatures like Ogres and demonic spirits of the Fade are noticeably tougher, but the best example is the nightmarish Brood Mother, who is explicitly stated to be the corrupted form of a normal female, and is a full-fledged boss monster. The Archdemon itself may also counts, though in its case the monstrosity makes it un-dragon-like instead of inhuman.
 * Gurumin: A Monstrous Adventure for the PC and PSP plays this straight most of the time, as each boss is usually smaller than the last. The first three bosses Bob, Mosby and Roger are all relatively large (after transforming), and then you get to the fourth boss who transforms... Into you. She suddenly has a ton of moments where she can block your attacks and counter with quick moves, whereas the first three are all a case of "Hit them when you get a good opening." The fifth boss is once again a giant monster, but you soon find that he's a complete joke and was only setting up for the real boss-- the Phantom Prince, who's roughly the same height as Parin. The trope's subverted as the final boss  But he's not the strongest enemy, actually. It's really the Bonus Boss, Blackbean: Prince of Destruction. He's the smallest boss in the game, coming up to about the same size as the game's standard Mook, roughly half of Parin's height.

Western Animation

 * In Ben 10, Ben's most used and most effective aliens are humanoid, while his least used (Brainstorm, Goop, Ghostfreak) are the most inhuman. Even Cannonbolt stood on two legs.
 * One could even compare the Highbreed, which is more or less a giant chalk white humanoid without a face, to the DNAliens, who are strange mixes among squids, brains and various other critters (and parasites besides), and see this.

Real Life

 * Somewhat Truth in Television: People with conditions that make them unnaturally large often have severe health problems. For instance, Joseph Merrick, The Elephant Man, had a pronounced limp, difficulty speaking, his enlarged right arm was useless and he had to sleep with care lest his enlarged head break his neck.
 * Partly the case, for example, with Andre the Giant, whose hulking size gave him massive sway in the wrestling world, but also crippled him as the years went by and eventually led to him dying at a young age. It's true in general of people with gigantism, who very often suffer health problems in later life because of the greater stresses on their bones and muscles compared to average-sized people.
 * Explained by the man himself, who saved is brother's life on the family farm when a tractor fell on him- Andre was able to lift it off with his freakish strength. He said of the incident that if it saved his brother, then his horrible pain was all worth it.
 * Similar tropes can be seen after a certain point in professional sports - you want to be big, but not too big. For example, the best NBA post players have generally been between 6'9" and 7'1". Most taller players have too high a center of gravity, too slow reflexes, and too many injury troubles to be superstars.