Many characters in Sonic X are significantly less useful than in the games, with Sonic, Shadow, Knuckles and Rouge being the only real exceptions. It was probably done for Cast Speciation, because everyone in the games can use Sonic's basic attacks and nearly keep up with him.
In Pokémon Special this happens with Whitney, oddly enough. Despite her and her Miltank being feared as one of the most difficult Gym Leader battles in all of Pokemon, she loses all of her fights in the Special manga. During her only battle where she comes off as a credible threat, she doesn't even use Miltank, instead using Cleffa and Igglybuff (who are Baby Pokemon, and in-game they are exactly as strong as they sound). She was fighting Suicune at the time, but it turns out that it was just one of Pryce's ice clones. In fact, she is one of the only Gym Leaders whom Suicune doesn't challenge; it's assumed in-universe that this is because Suicune never considered the possibility of her being worthy. Basically, she becomes the Butt Monkey of the Johto Gym Leaders.
This happened to King Dedede in Kirby of the Stars. In the games, he could hold his own against Kirby, and came close to winning against powerful adversaries, most notably Nightmare in Kirby's Adventure. In the anime he relies on the Monster of the Week to take care of Kirby, and while in the games Dedede would fight Kirby himself after they failed, in the anime he just gives up after the episode's monster is defeated. His flying and swallowing powers are gone, and he only really uses his hammer to hit Escargoon for comic relief. If the Monster of the Week disobeys Dedede and attacks him, Dedede would probably just run away and cry for help.
Kirby himself is a victim of this to some extent. While Kirby is usually more powerful with a copy ability (except for a joke ability like Sleep), it is still possible to clear some stages and bosses in the games without using a copy ability. In the anime, Kirby is pretty much a punching bag until he gets a copy ability.
Meta Knight suffers from it too. Most of his powers: his tornados, his ability to fly, use of electricity and fire, are removed from this version, and most battle's he's in will either have him undergo The Worf Effect, or leaving Kirby do the work so he can get stronger.
While Touhou Ibunshu significantly lowers everyone's power level compared to the original notoriously overpowered Touhou cast, none get hit harder than Reimu. She goes from one of the most broken players in a game that already begins broken to someone who can't use any magic at all, frequently finding herself utterly useless and repeatedly asking herself what she's doing among all these mystical beings in the first place.
In The Jungle Book by Rudyard Kipling, Kaa the python is a powerful Badass and one of the oldest, wisest, and most feared animals in the jungle. His wisdom and hypnotic dance make him one of Mowgli's most powerful allies, and even Bagheera and Baloo are wary about going to him for help. In the animated Disney adaptation, Kaa, while still dangerous, is a Butt Monkeyvillain. He becomes even more of an Adaptational Wimp in the sequel, in which he is beaten with a stick by Shanti's little brother.
Akela and the wolves also get this, as in the opening of the film they give Mowgli to Bagheera because they feel that they cannot protect him from Shere Khan. In the book, Shere Khan himself comes to demand the child, and the pack stands up to him. Akela remains an important ally to Mowgli until the wolf's death.
Let's not forget Mowgli himself, who in the original story survived the jungle's and the man-village's dangers by being too Badass for either of them to be able to handle.
Takua in BIONICLE: Mask of Light. In the Mata Nui Online Game, Takua flew as Kongu's second during the Gukko Force's attack on a Nui-Rama Hive, led six Matoran in a hopeless defense of Kini-Nui against hordes of Rahi, and braved the depths of the Makuta's lair itself. In the Wall of History animations, Takua aided in the liberation of Le-Koro, stood up to an entire Pahrak swarm, and joined the Toa Nuva in their pursuit of the Bohrok-Kal. In Mask of Light... Takua constantly shies away from his duty and responsibility, panics in the face of danger, and abandons Jaller after getting scared by Makuta. It's not until he stands up to the Rahkshi in Onu-Koro that Takua finally shows off some of his former Badass qualities.
Kongu in BIONICLE 2: Legends of Metru Nui, a brave and serious captain of the Gukko Force reduced to a trembling coward when threatened by the Dark Hunters.
Sidorak in BIONICLE 3: Web of Shadows. In the books and comics, Sidorak was a fierce warrior king who led his Visorak hordes into battle. In Web of Shadows, Sidorak is a sniveling coward who always needs Roodaka to do things for him, and Roodaka even outright says that he would never dirty his hands with conflict.
All of the villains, sans Metus who was a weakling to begin with, in BIONICLE: The Legend Reborn:
Bone Hunters and their Rock Steeds: Short but deadly and merciless desert bandits riding on savage dinosaurs. In the movie, a bunch of weak Mooks who ride on dinosaurs that behave like horses.
The Skrall: Spartan-esque soldiers who have never been beaten in a one-on-one duel before. In the movie, they run around squawking like animals and fall to a single hit.
Elite Skrall: Same but more so — several times tougher than regular Skrall, even weaker and dumber than them in the movie.
The "Mighty" Tuma: In the stories leading up to the movie, as well as in the novelization, he's a ruthless and Dangerously Genre Savvy overlord who came close to conquering the planet, and was only defeated because his size made him a slow fighter in close-combat. In the movie, he's a bumbling, full-of-himself brute who considers pushing over a much smaller opponent a deed worthy of celebration. He is defeated by the hero battering a wound on his back, which he left entirely unprotected every time he turned his back on him to bask in his own glory.
Films — Live-Action
Mina Murray in the many film adaptations of Dracula, but most drastically in the Bela Lugosi version, which she spends most of weeping hysterically. In the book, she plays an active role in the defeat of Dracula and is the Team Mom. The one time in the novel in which she gets hysterical is for a very good reason.
In the show earthbenders could do things like create and manipulate relatively simple objects like stone carts, they could open holes in the earth to swallow foes, or bring up pillars of stone under an enemy's feet to launch them into the air by themselves. In the movie, they just chuck rocks - and it takes multiple Earthbenders a great deal of effort to move even a relatively small rock.
In the show, even beginner firebenders could easily create fire from their own body heat. In the movie only the greatest firebenders are capable of this - for most of them they can only use their bending abilities if there's an existing source of flame, like a campfire or a torch.
In addition to earth and fire getting nerfed, bending as a whole takes a lot longer to do, requiring very long series of movements to do just about anything. This was actually intentional per Shyamalan's... unique interpretation of how bending works. In his words, he imagined bending as being a lot like dancing with the bender doing multiple moves to build up his power before finally releasing it all at once. Not only is this completely backwards from the cartoon, where any attack could be performed with simple punching and kicking movements, but it also makes elemental bending seem very inefficient - normal people might not be able to shoot fire from their hands, but they can chop a bender's head off while he's performing his intricate multi-part interpretive dance about burning people.
Female characters got hit particularly badly by this. The Kyoshi Warriors from the original cut of the film were Adapted Out save for a brief mention in an Earthbending village. Katara is also shown to be significantly less powerful and less skilled in the film than she is in the TV series. The most notable example is her fight against Zuko in the Northern Water Tribe city. In the cartoon she came close to winning the fight, only finally losing when the sun came up, thereby strengthening Zuko's powers and weakening hers. In the film she gets reamed in what is easily the most one-sided Curb-Stomp Battle in the entire movie.
In the video game franchise, Mortal Kombat, Stryker was one of the best characters in the game. In Mortal Kombat: Annihilation, he gets killed offscreen, with the only mention of him even existing being an offhand remark by one of Shao Kahn's henchmen about how easy he was to kill. Rumor has it this was a direct response to the fandom's widespread dislike of the character brought about by his status as a Tier-Induced Scrappy.
Donald Gennaro in Jurassic Park. In the novel, he goes along with Muldoon to catch the Tyrannosaurus, manages to fend off a Velociraptor attack, intimidates a ship captain with Technobabble, and survives to the end. In the film, he becomes a Dirty Coward who dies a particularly embarrassing death. (Remember the guy who got eaten by the T Rex while he was on the toilet? That was Don.)
Dragonball Evolution, otherwise known as "the American, live-action Dragon Ball movie" subjects Goku to this trope. In the original Dragon Ball, Goku was a One-Man Army, more or less impervious to bullets, able to leap tall buildings In a Single Bound and then punch those buildings into rubble. In the film, he... well... can't. It gets worse with regards to the Kamehameha. In the cartoon, Goku managed to learn the Kamehameha after seeing it used one time. The film, on the other hand, spends a significant portion of its run-time with Goku struggling to figure the move out.
Where to even begin with 1993's Super Mario Bros.? The entire Koopa family as shown in the film are changed from badass fire-breathing turtle dragon sorcerers into fairly average humanoids, with the only strange thing about them being that they evolved from dinosaurs rather than apes. Koopa troopas and goombas zigzag into Adaptational Badass by changing into big burly guys with tiny heads, but then it gets subverted when you see them in action. Mario and Luigi never once jump on someone's head or change into tanukis. There's even a brief moment at the end where the movie teases us by having the Devolution Device used on King — er, excuse me, President Koopa, turning him into a Tyrannosaurus Rex. Sadly, a T. Rex still isn't quite as badass as a fire-breathing turtle dragon, and he ends up being an Anticlimax Boss anyway, getting devolved into primordial ooze in short order.
In most adaptations of Frankenstein, this trope happens to Frankenstein's Monster. In the original novel the monster is an agile, extremely strong, and highly intelligent Genius Bruiser who is literate and able to speak eloquently, while in most adaptations he is grunting Dumb Muscle. This is the result of Lost in Imitation, with most adaptations thinking incorrectly that they're playing true to the source material, when they're actually imitating another adaptation.
While most of the characters in Street Fighter lack some of the powers of their video game counterparts, Dhalsim is an especially notable example. In the games, he's a stretching, fire-breathing yoga master. In the movie, he's a bullied lab technician with no powers. Supposedly, he was going to get his powers in the sequel, but it was never made. Chun-Li at first appears to be a case of this trope, being a seemingly regular news reporter who hides behind her two bodyguards. Of course, it's later revealed to have been Obfuscating Stupidity.
In Dungeons & Dragons beholders are Eldritch Abominations big enough to bite a human in half and with enough magical power that they shouldn't ever need to. Their single main eye renders all magic in its line of sight inert, and their numerous eye-stalks are capable of firing magic energy beams that can mind control, petrify, disintegrate, or even just outright kill enemies just by looking at them. They're easily among the most dangerous creatures in existence, xenophobic to the extreme, and highly egomaniacal. In the movie, beholders are downgraded to minor watch dogs for the villains.
It should also be noted that this was before Mace Windu crushed his chest.
Iron Man 3Zig Zags this trope. The Mandarin seems like a ridiculously Exaggerated example when it's discovered that he's really a drugged out actor playing a part. This is then Subverted when you discover that the actor is a Body Double for the real Mandarin, a terrorist mastermind who holds a Badass card. At the same time, while The Mandarin is still equally Badass his powers aren't exactly the same - in the comics he drew his power from magical rings, in the film his powers come from genetic manipulation.
Sideways may count as an example himself. His presence in Transformers Armada saw him as a Double Agent that could switch alliances between the Autobots and Decepticons at will, easily deceiving them and also a quite capable combatant in the anime and the toyline. In the movie, he's a Dirty Coward with no lines and spends his entire time running from Arcee and her sisters before getting sliced in half in his car mode by Sideswipe. (That Audi that transforms once to ram through a building and escape before this happens? That's Sideways).
Transformers: Dark of the Moon does this to Wheeljack. While both versions are a cunning Gadgeteer Genius; Wheeljack in The Transformers could still do well in battle. In the movie, Wheeljack, now referred to as Que, ends up getting captured by the Decepticons in the final battle in Chicago and begs for his life before getting unceremoniously shot to pieces.
The Harry Potter films have been accused of doing this to Ron. For example, in the first book, Ron and Harry are trapped by a monstrous plant, and Hermione has to save them; she panics so much that she forgets about her powers, and Ron is the one to angrily remind her what she can do. In the film Ron almost dies because he panics, and Hermione basically figures out how to save him herself, all while acting relatively calm.
The Lone Ranger: John Reid is somewhat less of a badass compared to his radio and TV versions. Justified as most versions of him are a Texas Ranger before donning the mask while this one is a City Mouse lawyer.
V for Vendetta turns Gordon Dietrich into a chubby, middle-aged comedian (played by Stephen Fry, no less) when he was a younger, more physically imposing career criminal in the book. Then again, the film also has him defying the Party's laws by hiding banned books and films in his house, and openly mocking Chancellor Sutler on his show (which he is eventually executed for), which is far more badass than anything the character did in the book.
French comic book series Les Profs (The Teachers) is about a cast of quirky, but overall competent high school teachers (except the lazy one who keeps finding new ways to avoid giving lectures). In the movie of the comic, they become the worst teachers of the whole French Educational system and are specifically selected as such (for instance the Napoleon-obsessed History teacher becomes a teacher wannabee who keeps failing at entrance exams because Napoleon is all he knows about history).
Doctor Watson in many adaptations of Sherlock Holmes. In the books, he was a very sensible and sharp-minded decorated ex-military Combat Medic who demonstrates through his writing that he possesses keen attention and memory, who began his adventures with Holmes while in his mid-20s, is described as strongly built and square-jawed, is portrayed as a man of action who was handy with a revolver and notably more violence-prone and confrontational than Holmes, and who more often than not insisted that Holmes take him along on dangerous missions as backup or confronted Holmes hotly regarding the latter's unhealthy habits or antisocial behavior. On film, particularly the Basil Rathbone/Nigel Bruce series from the 1940s, he is generally a timid, obedient, ineffectual old fool who is usually fat, feeble, and many years older than Holmes with a hero-worshippy, anxious, speak-when-spoken-to demeanor. Some later adaptations (like the Granada TV series) tried to undo this, but his image in the broad public consciousness didn't really get revamped until the 2009 movie inverted this trope with a vengeance.
King Renly Baratheon in Game of Thrones. His book counterpart is a big, burly man with an enthusiasm for jousts and battle note Though he's mentioned as having no talent for battle at all, and enters jousts only to be beaten by better fighters and have a good laugh about it. In the show he's of smaller stature and has no love for violence.
From The Tick we get Fish Boy: lost prince of Atlantis. While we don't really see him in action in the cartoon we do know that he is at least classified as an actual superhero. In the live-action series he's downgraded to a milquetoast of a sidekick, who is constantly physically and emotionally abused by his hero, The Angler.
A number of Superman games, including the notorious Superman 64 suffer from downplaying Superman's powers in order to provide some risk for the player character. The problem is that they go too far into this trope and also remove the fun of playing as a ridiculously powerful character like Superman.
In the 16-Bit game Death Of Superman (based on the eponymous comic arc and Reign Of Supermen arc), Supes can be hurt by simple mooks with baseball bats.
Mortal Kombat vs. DC Universe reduces Superman's power level to prevent him from simply punching the Mortal Kombat fighters into orbit. The game justifies it using Superman's somewhat well-known weakness to magic.
Injustice: Gods Among Us plays with this trope a little. While characters like Harley Quinn and The Flash can hurt characters like Superman and Wonder Woman simply by punching them, super attacks show them at vastly different power levels (with Superman literally punching his opponent into space) even if they all do about the same amount of damage. Plus, story-wise, things only shift in favor of the heroes after the non-evil Superman is brought to the Injustice universe.
In the novelization of Doom 3, Councilor Swann is a typical Dirty CowardObstructive Bureaucrat corporate suit, while in the game he was an aversion of the stock character type, being actually competent and decisive.
The OAV of Fire Emblem Mystery of the Emblem does this to both Marth and Caeda. In the game, Marth was somewhat naive with a soft touch but eager to get the job done. Caeda was a devoted Action Girl who was fighting beside him right from chapter 1. Here, Marth is a mopey Emo Kid who cries at the thought of killing a dear while poor Caeda catches the Distress Ball and instead of rushing in to fight the pirates, tosses Marth the sword and sits in the back lines folding bandages.
Similarly, Green Lantern's ring was used in a much more limited fashion in the early seasons. Eventually, this too changes with the writers lampshading the matter by having his fellow Lantern Katma Tui lecture him on his unimaginative use of the ring.
There's also Wonder Woman very belatedly getting the full powers of her lasso unlocked, finally bringing her up to par with her standard comics incarnation.
Martian Manhunter also had it bad, he is the one who gets the most beatings that everyone else, and usually gets knocked out more often.
While Aquaman has always been at his best around open water, he still has super strength and resilience even on land. The writers for Super Friends, apparently as an attempt at Cast Speciation, downplayed all of his non-aquatic powers to the point of him being almost completely defenseless out of water. This led to the popular belief that he was the most useless member of the team, with many viewers considering him The Scrappy.
In Challenge of the Superfriendsthe Scarecrow was one of the more useless members of the Legion of Doom, with his only shown ability being to summon crows which rarely did more than annoy the Superfriends. In the comics, however, he is usually written as a considerably more dangerous villain, being adept at physical combat (especially when armed with his scythe) in addition to his "fear gas", a hallucinogenic nerve gas that is enough to render most people a sobbing wreck with just one whiff. His more threatening characterization was restored in the last iteration of the show, The Super Powers Team, but even then he's not quite as badass as he is in the comics.
More generally, the "no punching" rules imposed by the Moral Guardians of the time ended up making many of the heroes and villains look unusually ineffective. Hawkman and Solomon Grundy were probably the hardest hit (pun fully intended) given that they were all about melee combat in the comics.
Wolverine and the X-Men does this to several characters who, in their portrayal in comics and elsewhere, would not allow Logan to steal the spotlight as completely as he does in this series. Storm becomes more timid and prone to getting a Tap on the Head to keep her from easily winning fights, Jean gets the sadly usual treatment of existing solely for the sake of the threat of Dark Phoenix and is reduced to living MacGuffin, and Cyclops loses all leadership quality in favor of Wangst. In one episode, the entire team except for Wolverine is taken down in seconds by Silver Samurai and some goons. They do get a few moments of being the X-Men you know and love, but they're sadly few and far between.
Magneto in general tends to get this. He's almost never as powerful as his comic counterpart. Ironic, since he rarely actually gets into any direct confrontation with the X-Men outside the comics.
Simon Belmont in Captain N: The Game Master. Granted, around 1990 the Castlevania games hadn't given Simon much in the way of character development, pretty much a Badass Normal who defeated Dracula twice in the Nintendo Hard Castlevanias using only a (at that point, not all that special) whip. In the TV series he was reduced to The Drag-Along. It's hard to believe the bumbling, narcissistic Simon of the TV series could possibly have gone up against someone as threatening as Dracula and even survived, much less triumphed.
Dracula, too. The existence-threatening Big Bad of the series got turned into a dork in a banana-yellow tux who could be stopped just by winding him up like a top and spinning him back into his coffin.
This also applies on some level with Mega Man and KidIcarus. Mega Man does have plenty of strength, but it's still quite a step down from having an arm that turns into a gun and the powers of the Robot Masters he defeats. Kid Icarus' has the marksmanship of his trademark arrows reduced to comical proportions, and on occasions where they do hit something, they do things like giving wolves balloons instead of winning fights.
The villains got hit with absolutely astonishing Villain Decay as well as this trope. In Punch-Out!!, King Hippo was one of the harder boxers in the game, but was easy to beat once one learned the strategy. As Seanbaby put it, his usefulness in the cartoon was "limited by cartoon censors who didn't allow punches". Eggplant Wizard goes from being a boss that can turn the player into a vegetable to merely making jokes about the subject and displaying no sorcery prowess whatsoever. The biggest victim had to be Mother Brain; going from the all-powerful Big Bad of her game to an Ineffectual Sympathetic Villain that never truly got the better of Kevin and the other denizens of the Palace of Power.
Later games struck a middle ground, where he's generally competent but almost always gets pushed aside by the game's final boss, which is usually something he built / freed that went out of control. While as buffoonish and comical as ever, he has actually posed even more of a genuine threat and eventually regains the role of final boss.
Tails. In the games, he's one of the fastest and strongest characters. In the TV adaptations, he's usually all but useless when it comes to fighting.
Sonic SatAM was the worst offender when it comes to Tails' treatment. In order to give its original charactersmore screentime, the show changed Tails from being Sonic's sidekick and main ally to a Bratty Half-Pint who was always made stay home while the others fought Robotnik, and the few times he ventured outside, he tended to play the role of Distressed Dude. You would that think the writers of an adaptation created solely to promote a video game series would have the common sense to give the spotlight to the source material's main, playable characters, but alas...
The Beyonder in Spider-Man: The Animated Series. It may not seem like that because of how powerful he still is, but that's because the comic version is so powerful that even if incredibly nerfed he is still more powerful than any other character. The comic version is revealed to be an entire alternate universe in human form. His powers can basically be summed up as "he decides reality will be a certain way, and so it is." The show Beyonder is, again, the most powerful character in the series, but his power can be exhausted with overuse, and when depleted, he was apparently in real physical danger from Man-Spider. Apparently he is comic-level powerful when in his own dimension, but again, comic Beyonder is his own dimension and is all-powerful wherever he is. While far from a Superfriends Aquaman level of suck, we at least go from standing alone at capital-G-God tier to coming down to... well, less.
Lava Lord, from Mighty Max. In the original toyline Lava Lord is a dangerous villain who is more than capable of fighting on his own thanks to his Fireforce Sword, which can control fire and heat. In the cartoon he relies 100% on his Humongous Mecha, Magus, and seems to be pretty much useless without it.
Most of the cast of Young Justice, which according to Word of God was an intentional effort to make the massive superhuman cast manageable. For instance, Superboy lost his Telekinesis, flight, ice breath, and Eye Beams, while Miss Martian lost her super-strength. And even without it, Miss Martian still managed to be one of the most scarily-powerful characters in the entire series.
Lolth herself - as in, the dreaded spider-goddess of the drow - appeared in the cartoon version of Dungeons & Dragons, where she seemed to be an ally of Venger; she first appeared as a beautiful elven maiden, who lured the heroes into a dark cavern, at which point she quickly assumed her true, demonic form and dumped them into a subterranean web. Venger appeared briefly to gloat, and then left her to deal with them, but she proved far weaker than she was in any other setting, defeated rather easily after the heroes managed to sever a web and causing her to plummet helplessly into the abyss below.