There's probably something symbolic about the tendencies of a villain to turn into a snake. In the West there is a strong established symbolism for snakes from The Bible (most famously the serpent in the Garden of Eden), Norse Mythology, and there's Orochi in Japanese mythology. Adding onto this, there is the fact that when walking in the wild, it's generally not a good idea to stop and pet a snake.
Of course, the result of all that symbolism is that when a villain turns into a snake there can be no doubt about the evil nature of a foe, and it gives The Hero a comfortably scary and suddenly less human foe to kill.
Villains do love their symbolism and cliches, however, so despite how often this trope tends to end with the villain chopped up on the hero's sword, (usually rather easily too) villains everywhere still love the order Squamata because, of course, Reptiles Are Abhorrent. Think about it for a second: how often have you ever seen a villain turn into, say, a bear, a rhino, a lion, a koala, or something else that would actually be useful?
This trope is not strictly limited for the bad guys. However, when it's the good guys doing this, they typically turn into dragons instead of snakes.
A specific variant of One-Winged Angel. If you're looking for video game characters being shown as grotesquely oversized compared to their surroundings on an overview map, that's over at Units Not to Scale.
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A drug PSA features a drug dealer who gets more snake-like towards the end.
Anime and Manga
Orochimaru from Naruto probably takes inspiration from his namesake Orochi and does this in the series. He gets taken out by a certain Ineffectual Loner after said loner's Face-Heel Turn. Though his chances as a snake were probably better than a drugged-up old man who was near-dying, and he apparently didn't have a choice in the matter.
Rather, Orochimaru takes his inspiration from the original Orochimaru, who likely gets it from his namesake Orochi.
Later, he takes on a Hydra form to fight an attack called Susano, named for the god that killed the eight-headed serpent Orochi. It doesn't go well for him — he was mythologically doomed to lose that fight.
In contrast to the above, Kabuto/Kabuchimaru turning into a snake let him escape a trap and kidnap someone on his way out. He later "evolves" this snake form into an Evil Counterpart to Naruto's Sage Mode, changing from a snake to a dragon.
A Monster of the Week from InuYasha does this. Notable as he actually put up a pretty decent fight while in humanoid form, (thanks to a magic trident) but got carved up like a Thanksgiving turkey post transformation. Beyond lame.
That's technically a reversal; said monster was a water snake spirit who stole the trident from the local river goddess, so he didn't change into a snake, he changed back from his assumed human form.
In One Piece, Boa Marigold and Boa Sandersonia turn into half snakes (a King Cobra hybrid and an Anaconda hybrid, respectively). They fare pretty well against Luffy…up until the point where he stops holding back.
In the first episode/chapter of Busou Renkin, one of Kazuki's teachers morphs into a cobra-like creature. He manages to swallow Kazuki's sister Mahiro whole, but gets thoroughly dismantled by Tokiko without even getting a chance to attack. Laaaaaame.
Umineko no Naku Koro ni: During the climactic fight at the end of the first season, Beatrice summons meta-Kinzo, who immediately gives an Evil Laugh, turns into a dragon, and tries to eat meta-Battler's face, only to be stopped by a non-magical explanation and blasted to pieces by a multitude of flying spikes that accompany Battler's signature dramatic gesture.
Inverted by G Gundam's Cobra Gundam, which starts off as a Naga and can separate into a humanoid Humongous Mecha and a mechanical snake, operated by the pilot's pet snake.
Claymore has Ophelia turn into an Awakened, resulting in her becoming a massive snake-like being with lots of blades. At first it helps against Clare, until she realises it herself by seeing her reflection, after which she has Clare to play a game of kill or get killed.
Near the end of the 2003 anime adaptation of Fullmetal Alchemist, Envy turns into a giant serpent in order to cross the Gate and kill his father, Hohenheim. He ends up getting stuck in Germany, where he's captured by the Thule Society and used as a giant Ouroboros to activate their transmutation circle and open a portal to Amestris. However, this process does accomplish the one thing he really wanted: killing Hohenheim.
An inversion in Yu-Gi-Oh! in the Waking the Dragons arc: the secret of the three dragons is that they were actually knights transformed by Dartz in an attempt to weaken them. Yami Yugi transforms them back into knights in order to defeat Dartz.
Yasha from Cutey Honey turns into a giant monstrous lamia in her final battle against Honey. She puts up a decent fight due to her impenetrable scales, but it turns out gravity combined with the spikes at the bottom of Yasha's lair are enough to tear through her scales.
Divine Ancestors in Campione! have the ability to gain a massive increase in power by transforming into their primordial form, a dragon or serpent. This comes at a cost as doing so sacrifices their eternal youth and hampers their ability to regain their lost powers.
In Part 7 of Jojos Bizarre Adventure, there is a Stand called "Scary Monsters" that allows its user Dr. Ferdinand and, later Diego Brando, to transform himself and anyone he injures into dinosaurs.
The Munchkin Conan expansion has the monster modifier "...That Turns Into A Giant Snake." Rather amusingly, you can put it on a giant snake. The card specifically says it turns into a different giant snake.
The Marvel ComicsFear Itself crossover ended with the Serpent, a humanoid Asgardian god whose name is more of a nickname/title due to his nature, abandoning his human form and all if its powers to turn into a giant snake that is easily sliced up by Thor for no apparent reason.
The Incredibles comic arc, City of Incredibles has Shifty taking on the form of a dragon after having his shapeshifting powers enhanced by a superpower virus, only to be taken down by the family after they powered up on the same disease.
In the video for the game Dragon Strike, as well as its tie-in Marvel comic, Teraptus turns into the giant dragon Darkfyre.
In the Omega Men comic book, turning into a giant snake was Demonia's only real superpower. It did, however, make her quite tough in a fight.
The Last Temptation graphic novel by Neil Gaiman and Alice Cooper (yes, you read that right) has The Showman turn into a giant snake at the end, but he doesn't actually attack the hero. He just messes with his head a bit, and scares him away. If this story is meant to tie into Cooper's song "Welcome To My Nightmare", (which many of his fans believe it is) then it was an entirely successful head-mess, because by then, Stephen is a Serial Killer.
Dragons in Gold Digger all have the ability to disguise themselves as humans, and several dragon characters have in fact been Mode Locked in human form. Generally the dragon population is divided on the opinion of whether a race of talking mammals like humans is adorable ("They're like little talking bunnies! I want to play at being a bunny too!") or disgusting ("They're like little talking rats!").
In the Vampirella story "She who Waits" the Cobra Queen can transform into a giant cobra.
In Disney's Aladdin, Jafar transforms into a humongous snake after gaining sorcerer powers from the Genie. He actually has Aladdin and company on the ropes for quite a while until Al tricks him into trying to gain even more power by using his last wish to become a Genie, and Jafar is sealed inside a new lamp. In hindsight, Jafar should have stuck to the reptile form.
An unintentional example in Bartok the Magnificent: the villainess is turned into a dragon after downing a potion.
The Incredibles: The climax of the City of Incredibles arc of the comic has Shifty turn into a red dragon after being enhanced by the superpower virus, to take down the Incredibles and mutiny against his fellow supervillains.
In The Return of Hanuman, Rahu and Ketu's henchmen turns into their true forms; which are dragon-like creatures, in order to snatch Rahu and Ketu's snake staff from Maruti. They failed because Maruti tricked them and eventually their necks gets twisted and tied up.
In Dreamscape, villain Tommy gets the bright idea of turning into a snake-like monster, quickly resulting in his decapitation. By the President of the United States, no less. And not a President Action, either, but an old, overweight, out-of-shape President Target who hits him from behind with a shovel.
At first, Thulsa Doom of the first Conan the Barbarian movie seems to be doing this for no reason at all, but then subverts the expectations of the audience by fleeing through a hole in the wall made for a snake and getting to safety. (It's also in keeping with his character so far as he seems to have a thing for snakes: his religion worships them, he has a pet snake that Conan kills, he has snake arrows... no, really.) Thus, not lame.
In The Mummy: Tomb of the Dragon Emperor, the Emperor turns into a three-headed dragon, kidnapping Lin and managing to evade the heroes. He doesn't return to that shape, though. Later on, he shifts into another giant animal, and was similarly unbeatable. In fact, it's safe to say that had he not been goaded into a fair fight, he'd have succeeded.
Freddy Krueger in A Nightmare on Elm Street 3: Dream Warriors turned into a serpentine creature to swallow a dreaming teen whole. He managed to gulp down her legs when Nancy appeared to haul her back out of his mouth. This scene could be considered foreshadowing, as we later learn that he absorbs/eats the souls of his victims to grow stronger.
In the 1973 b-movie Sssssss, a Mad Scientist turns his lab assistants into king cobras.
In The Silmarillion, Sauron at one point becomes a snake in an effort to defeat Huan, the Hound of Valinor. And no, it wasn't any more successful than any of the other forms he tried.
The Lady of the Green Kirtle does this in The Silver Chair. Lewis was definitely being symbolic in this case, but it's still a dumb move, especially when confronted by a Stupid Good hero who Wouldn't Hit a Girl but has no problem cutting a giant serpent in half. Lame. The same villainess apparently used her serpent form to assassinate the queen of Narnia, some years previously, and had pulled it off without a hitch, though.
In another incident in the same series, a protagonist is turned into a dragon for being greedy (again, symbolic). This was not as awesome as it seemed since he was apparently an old-style gangly ugly dragon, he had a metal ring digging painfully into his arm the entire time due to his increase in size, he could not talk, and was too big to ride a ship.
In the Chronicles of Prydain, when Evil Overlord Arawn, (based on the Celtic God of the Dead, who shares the name), master of terrible armies and incomprehensible Black Magic is finally confronted face to face, he promptly turns into a serpent and dies within a paragraph or two, managing only to kill the slightly less evil queen before falling to everyone's favorite assistant pig keeper. Lame.
Justified as he was trying to get the hell out of his invaded castle.
In The Death Gate Cycle, the ultimate evil beings are shape shifters that like to take the form of giant dragon-snakes, probably meant to resemble wingless Chinese dragons with a more snakelike head. In a final confrontation with the heroes one of them reverts into this shape, growing too large, and his head hits the roof of the room they're in, which is inscribed with death runes, thus killing him. Uber Lame.
Somewhat justified by Fridge Logic, as the chamber the Final Battle took place in was filled with magical energy that was anathema to the serpent- he was likely dying by inches from the moment he entered it (which he only did out of desperation).
At the climax of the first book in the series, the Evil Sorcerer antagonist pulls a more traditional version of this trope, turning himself into a giant snake, but he dies quickly because he'd already been fatally knifed in human form and the wound carried over.
A protagonistturns into a dragon during The Siege in the sixth book. He kicks a lot of ass (mostly off-screen, but considering what he's facing it's still badass). It's only a partial success because he ends up missing in action. The seventh book reveal why: he was defeated and captured by a real dragon. And not the lame kind mentioned above, a "one person in the entire series has defeated one in battle" dragon.
The universe of Steven Erikson and Ian Cameron Esslemont, best known from the Malazan Book of the Fallen books, features the Soletaken, who can change ("veer") from humanoid to beast form. Some Soletaken veer into draconean [sic] form, which is powerful, but somehow never seems to really work out in the story. Examples:
Gardens of the Moon (book one): Silanah (a true dragon, not a shapeshifter) and four Tiste Andii Draconean Soletaken face off against Raest, the Jaghut Tyrant. He chases them off.
Gardens of the Moon (book one): Anomander Rake veers into draconean form and flies over Darujhistan, but he doesn't accomplish anything, and sembles back to humanoid form to fight a Malazan demon.
Memories of Ice (book three): Anomander Rake veers into draconean form to harry a horde of humans, which should be a trivial task. But one of them, Anaster, somehow poisons him, and Rake shies away. Then he takes humanoid form in order to kill some measly witches.
Reaper's Gale (book seven): In the prologue, Scabandari Bloodeye, in draconean form, fights two Elder Gods and is killed.
Reaper's Gale (book seven): Menandore, Sheltatha Lore and Sukul Ankhadu, in draconean form, face two measly humans (Quick Ben and Hedge). All three end up badly wounded and getting killed soon after.
Reaper's Gale (book seven): In the ultimate anticlimax to this book, Silchas Ruin (Anomander Rake's brother) faces (again) a bunch of measly humans and gets his butt handed to him.
Apparently the only time this ability was used successfully was in House of Chains when Osseric veered into a dragon to fly his son L'Oric out of the crumbling memory of Raraku.
In the Dragon Knight series, James actively avoids turning into his dragon form during battle unless he needs to intimidate his enemies. While he is big and scary, he's also extremely vulnerable and unable to move as quickly or nimbly as a human foe.
During a Shape Shifter Showdown in an early Discworld novel, at one point the wizard transforms into a snake. Granny Weatherwax transforms herself into a snake charmer's basket as a counter.
The Pirates of the Caribbean: Jack Sparrow book, City of Gold concludes with voodoo witch Madame Minuet fusing with her two new allies into a three-headed serpent to try and take down the young pirate.
This plays into the final Animorphs book. Rachel has snuck aboard Tom's ship, hoping to defeat his rogue Yeerk army. She transforms into a Grizzly Bear and he sends his minions after her after they have each changed into equally fearsome and tough creatures. Rachel has just enough of an opportunity during the battle to check on Tom as he hangs back and transforms - into a very small but very poisonous snake. After Rachel has defeated each of the minions, she bites Tom in half. She demorphs, exhausted, and another minion, transformed into a polar bear, kills her with a single blow.
Mister Monday transforms into a giant snake in the first book of the Keys to the Kingdom series, He ALMOST succeeds in killing Arthur, until Arthur gets Suzy to write on him with her fingers and a bottle of ink, causing him to be attacked by hundreds of text-destroying, acid-spitting Nithling-snakes.
... upon which said wizard breaks Snakyboy's newly re-acquired kneecaps.
In Curse Of The Wolf Girl the Big Bad's dragon really can turn into a dragon and manages to put up a good fight and has Malveria's army on the ropes. Of course there remains the issue that the reason there are no true dragons left is because Malveria killed them all years ago, and turned them into dragon-scale outfits. It ends about as well as can be expected when Malveria turns up to take charge personally.
Averted, as well as inverted, in the Harry Potter series. In a world where wizards can, with hard work, learn to transform into an animal that (arguably) fits their personality, Voldemort is: the last descendant of a man known for talking to snakes, an inheritor of this ability, member of a House whose symbol is a snake, commander of a giant serpent, owner of the pet snake Nagini, and wearer of a face repeatedly described as "snakelike". At no point does he turn into a snake. (Although events in Order of the Phoenix may confuse one into thinking so.) However, in Deathly Hallows, Nagini is at one point disguised as a human.
The novel Brothers in Arms inverts this trope: the Dark Action Girl Kitiara is trying to kill a dragon and her only chance to do so is when he is in his vulnerable human form, rather than in his nigh-invincible natural shape.
In the fourth Fablehaven book, Gavin, Kendra's "boyfriend," turns into the demonic dragon Navarog. He could have easily killed all the heroes then, but he decides to revert to his human form and capture Kendra, allowing the gay fairylike dragon Raxtus to bite him in two.
Played straight by a female Dark One at the conclusion of The Wizard of Sunset Strip. Probably a tribute to Sleeping Beauty, as the battle in question takes place at a reconstructed Disneyland.
During Galaxy of Fear, the heroic shapeshifter Hoole has at least two snake forms, one for stealth and the other suitable for combat. Bad things happen more than once when he does, notably while on Kiva. He once fights another shapeshifter, his own clone, and both take snake forms during their Shapeshifter Showdown. The battle is a draw and ultimately the deciding factor is another character going Spot the Impostor.
The Dragonlord Chronicles books from Mystara feature Sir George Kirbey, a seemingly human knight who is actually a sort of small were-dragon called a drake. Further, it is eventually revealed that the hero Teldin is a descendent of the dragons' gods, and he becomes one himself.
This was actually the end goal of the Mayor from Season 3 of Buffy the Vampire Slayer, which he'd spent a century working towards and built all sort of elaborate machinations around. He promptly gets taken out, of all things, by Buffy exploiting his love for his pseudo-adopted daughter. Oh, and a stockpile of well placed explosive, while a bunch of high schoolers mostly armed with crossbows and such fight off his minions.
But hilarious-aside from his undeniably evil aspirations, he's been characterized as a personable, mildly absent-minded guy who doesn't like swears. His only comment on his seconds-nigh doom? "Well gosh!"
In Chinese Paladin 3, one of the heroines turns into a snake to get an extra edge in combat.
On that note, in the "Kinda" serial of the 5th Doctor's reign, "The Mara" reveals its long-awaited true form to be a giant, laughable blow-up snake, which flails about for a few minutes before being defeated.
Kamen Rider Kiva had his own heroic example one year earlier - Emperor Form: Flight Style turns him into a winged dragon with an impressive Breath Weapon. There are two accounts of how he got it - in the Non-Serial Movie, it's forced on him by the villain, and he's unable to control it until he gets a cooldown-hug from Otoya. He reverts to normal until the villain uses his ownOne-Winged Angel form. In the actual series, it's just an extension of Emperor Form that he unlocks late in the series and has no problem controlling.
A variant occurs in the Red Dwarf episode "Polymorph" when a shapeshifting Emotion Eater turns into a snake. It works because it wasn't for combat reasons, but to freak Lister out (as he is scared of snakes). It helps make him even more scared, until the beast assumes its true form and drains him of his fear.
Inverted in Stargate SG-1: The Goa'uld are actually snakes that turn into humanoids (sort of).
More like Double Subverted or just Played With. Physically, Goa'uld are foot-long, water-dwelling worms with big fangs on a round mouth and a fin or frill below their heads - basically, snakes with Spikes of Villainy. However, they are also Puppeteer Parasites who take over the bodies of humans and other species. However, some Goa'uld ruled over humans as God Emperors, most of whom used some kind of totem animal as an icon, and the Big Bad for the first several seasons happened to use a snake as his icon. So there's a snake inside a human, whose Mooks are ordered to dress up as snakes...
The (already enlarged) Fenrakk spider is transformed into the Kardas dragon after the Toa Inika made him and his rider Vezon fall into magma. As a subversion, this was an involuntarily transformation on their part — it was the work of the Mask of Life, which tried to protect itself by upgrading its guardians. The Toa realize that if they kept on defeating it, it would only come back more powerful, so they freeze the dragon and Vezon in time with a special ammo and then take the Mask from them easily.
Makuta Miserix had a fondness for reptiles, so he shapeshifted into one whenever a battle broke out.
Inverted with Krahka: she tended to turn into a smaller snake to sneak away or avoid detection.
In the Robbie Williams music video "Radio", Robbie becomes something like this at the very end.
Mythology and Religion
Greekmythology example: the river god Achelous turns into a serpent to fight Hercules for the hand of a princess called Deianeira. He loses, and in one version Hercules rips off one of his horns, which was used by the nymphs to make the Horn of Plenty.
A bizarre example in many ways, the traditional Chinese fairy tale Legend of the White Snake has Madame White Snake forcibly morphed into her original form when she is captured by the monk Fa Hai.
In the 4th edition of Dungeons & Dragons a druid can take the Paragon path of coiled serpent, where you have the ability to scale up. what you do with this ability and whether you die or not is at the player's (or more realistically, the DM's) discretion.
Inverted by an NPC from the CarnivalRavenloft supplement, a snake familiar who had a Heel-Face Turn and abandoned her evil master after he turned her into an elf.
Also, many types of D&D dragons are depicted as being able to shapeshift into human form. Some even fall in love with humans.
Mystara has a race of small dragon-like creatures called drakes who can become human at will.
Back in 3.5, the spell Dragonshape lets you instantly turn into an adult red dragon. There's also the variant spells Least and Lesser Dragonshape, which turns you into a pseudodragon and a young red dragon, respectively.
Pathfinder on the other hand gives you a much more versatile spell, Form of the Dragon, with three variations for Medium, Large, and Giant dragons, and the user choosing which dragon to turn into. DraconicSorcerors get access to all three versions as bloodline spells.
In Vampire: The Masquerade the Followers of Set, one of the 13 main clans, have an entire set of powers related to becoming a snake, called Serpentis. The 1st power lets you immobilize someone with your gaze, the 2nd grows a razor sharp tongue to use as a weapon, the 3rd gives you armored scales, and the 4th turns you into a giant cobra.
Werewolf: The Apocalypse has a double-helping of this trope: the 1st Edition Players Guide introduced us to the Mokole, a race of were-saurians whose relationship to the sun is similar to the wolves' relationship to the moon, while the 2nd Edition Players Guide introduced the Nagah, a predominantly Asian and Middle-Eastern race of were-snakes who functioned primarily as assassins.
Chuubo, the titular character of Chuubo's Marvelous Wish-Granting Engine, has the ability to turn into a giant 108-meter snake at will. The name of that power? "It Probably Wouldn't Help." Sure enough, there is an actual law of physics in that game (backed by the rules) which states that turning into a snake cannot help you.
Happens twice in Der Ring des Nibelungen, to the same Leitmotif. In Das Rheingold, when Alberich is demonstrating the shapeshifting powers of the Tarnhelm, he first transforms into a dragon, and Loge pretends to be frightened. (Wagner's actual word, Riesenwurm, literally means "giant worm.") In Siegfried, Fafner has already transformed himself from a giant into a dragon. Since he has the Artifact of Doom, his fate is sealed.
The Dragonlord does this in all the various remakes of the original game, too. It was decided that the Big Bad transforming into a dragon as his One-Winged Angel form was more interesting than easily killing him and then having to fight his pet dragon that he inexplicably didn't sic on you first.
Eye of the Beholder 2 has the game's Big Bad Dran Draggore turn out to be a Dragon. Of course, the dragon is still vulnerable to the attack+ side-step maneuver.
The World Ends with You: The Final Boss first fights you in his human form, which is a tricky fight because his attacks are fast and unpredictable and can hit very hard. Then he turns into a giant snake, which is generally easier because its attacks are much slower and easier to avoid. Then he turns into an even bigger dragon and traps all your partners in bubbles.
No pity for Luna? Especially after she's forced into this due to the FMian Ophiuchus hijacking her? Thankfully, Geo beats her back to normal and Gemini slaughters the smug snake, but still...
On a side note, why wasn't Luna's hair as Queen Ophiuca/Ophiuchus Queen used as a weapon? It was visibly rotating!
The MacGuffin in Tomb Raider 2 was a magical dagger that would turn those who stabbed themselves through the heart with it into a dragon. Having said dagger removed would kill them. In the intro, the ancient emperor who originally owned the dagger is killed when he gets too close to a wounded monk, while the Big Bad of the game is killed when Lara shoots him a bunch of times, then pulls out the dagger.
King's Quest V: Absence Makes The Heart Go Yonder!. The showdown with Mordack involves him shapeshifting into various forms, which Graham must counter with some spells he learnt very conveniently about five minutes earlier. One of which is a dragon (defeated by turning into a rabbit, too nimble for the dragon to hit). Straight after, it's a snake (obviously countered by a mongoose).
Simon the Sorcerer. At one point, you partake in a "wizard's duel" (read: magical rock-paper-scissors) with a witch. Upon winning three rounds, the witch transforms into a dragon (breaking the rule she set at the start). This is completely innefective, of course - you just transform into a mouse and escape through the small mousehole in the wall.
Draconis and Abazigal from Baldur's Gate II: Throne of Bhaal revert to their true forms — adult dragons — upon being whacked sufficiently while in human form. While you still kill them, the 'partial success' part is because Draconis' dragon form is a challenge (or at least a bloody annoyance) and is widely considered to be an actually difficult boss. Abazigal, however... Isn't.
After being thrashed sufficiently by Isaac's team in Golden Sun, antagonists Saturos and Menardi transform into a single two-headed dragon. While the Fusion Dragon is a more powerful boss than either opponent separately, team Isaac still wins. In the sequel, the final boss is actually another fused dragon comprised of three of the heroes' parents.
In all these cases, the dragon shape really is far more powerful (especially in the case of the Doom Dragon), but results in a nasty case of Antagonist RROD when it wears off. A line of Saturos's before his transformation suggests he knew about this which would explain why he and Menardi were Driven to Suicide after their defeat as the Fusion Dragon. In TLA, everyone who transformed was explicitly stated afterwards to be too weak to withstand Mars Lighthouse's environmental hazards which is how Agatio and Karst died. The heroes' parents were saved by Deus ex Machina.
Avaritia, the leader of the Black Knights in 11eyes, stays on the sidelines for most of the game. Come the final fight, he transforms into a gigantic black dragon. Kukuri and Shiori face him down, and, though it takes a hell of a lot of power from Shiori, they just barely end up victorious.
In a heroic version, Aelia of Valkyrie Profile turns into a dragon briefly during her Finishing Move to fire a giant beam at enemies. It only partially works because it's a bit weaker than most anyone else's finishing move, and is even less useful when you consider the one hit does little to help the Combo Meter.
One of the minor villains in Arcanum: Of Steamworks & Magick Obscura is Kraka-Tur, a human who, tired of being bullied for his weakness, killed Arcanum's last dragon by feeding him poison and used his blood in a magic ritual which turned him into a dragon, a crime for which he was banished. While the transformation gives him impressive strength and fighting prowess, he still has the personality of a coward.
The top-tier transmutation spell in Dungeon Crawl is Dragon Form which turns the player into a dragon. Whether or not it helps depends on the player's skill. On the one hand, the player is rendered vulnerable to cold and loses the benefits of any magical items other than rings. But the player is also a dragon, and, capable of breathing fire on enemies and, with sufficient skill in unarmed combat, tearing even other dragons to shreds in hand-to-claw combat.
This is the main character's signature ability in the Breath of Fire series. He usually starts out as a human from a Lost Tribe with the power to transform into dragons, and throughout the game he gradually acquires bigger and more powerful dragon forms to use. In the fifth game he only has a single transformation and has to avoid overusing it.
Major antagonists in the series usually have a similar power: Emperor Zod in the first game, Teepo in the third, and Fou-lu in the fourth all fight the party in their dragon forms, the last more than once. The situation in the fifth installment is more complicated, but the technical final boss and the Bonus Boss are still both dragons.
The Manaketes in Fire Emblem provide both heroic and villainous examples of this in the dragon variety, and are a force to be reckoned with for those not fortunate enough to have a dragon-slaying weapon on hand. They're not so much becoming dragons as they are reverting to dragons, though, having been forced to take human forms to avoid losing their minds.
In later Dragon Quest games, the Be Dragon spell, or Puff as it is now known, appears a handful of times. It allows the user to transform into a fire-breathing dragon for a few turns. Very effective, especially in Dragon Quest III where it was introduced; in that game it's the only reliable way to kill multiple Liquid Metal Slimes in a single battle.
In EarthBound, the dragonite item turns one character into a dragon and damages every enemy for a ton of damage.
One of Liu Kang's finishers in the Mortal Kombat series is to turn into a dragon and bite the opponent in two.
Likewise, Shang Tsung's animality from Mortal Kombat 3 had him turn into a glowing green giant cobra that swallowed the victim whole.
In Dragon Age: Origins, Flemeth turns into a dragon and is one of the hardest fights in the game.
She does it again in Dragon Age II, although that time she helps the player.
In Mystaria: The Realms of Lore, one character, Ashe, can change into a dragon at will. While he doesn't have quite the variety of abilities as a dragon as he does while human, he's so ridiculously powerful as a dragon that there's almost literally no reason not to be a dragon at all times.
In Guitar Hero: Warriors of Rock, this is Casey Lynch's powered-up form.
In Brain Dead 13, there is a giant snake that can wrap around Lance and eat him up at the maze if you're not quick enough.
In the PS1 strategy RPG Saiyuki: The Journey West, one of the characters is a dragon princess who can transform into her true form in combat. One of the other playable characters can turn into a monstrous turtle in the same way. (And if fish scales count, yet another of the playable characters transforms into a fish monster) On the villain's side, there's a dragon queen and a snake man, who you fight in both forms in separate battles.
Homestuck: When cherubsmate, they each grow into a serpentine form that is one astronomical unit (149 ,597,871 km) in length. Then, in great hatred, they wage an epic battle around a black hole, and they conclude this battle with a position shaped like AURYN from The Neverending Story. It is implied, then, that this form is at least incorporated into Lord English's One-Winged Angel.
As the quote at the top of the page indicates, the Evil Overlord List advises against this.
Lung of Worm slowly metamorphoses into a dragon as he fights.
Malchior in Teen Titans, though he didn't so much transform into a dragon as he was transformed back to his real form. It still doesn't help him much.
Done again when Beast Boy gets an Evil Twin. It transforms into a cobra, just to show it's evil by using a form Beast Boy never would.
The classic Betty Boop cartoon "Snow White" ends with the Evil Queen turning into a dragon after her magic mirror blows up, and promptly being taken down by Betty, Koko the Clown and Bimbo the Dog. Considering that the highlight of the cartoon involves Koko being turned into a ghost-like creature singing "St. James Infirmary Blues" with the voice of Cab Calloway, this can be seen as the least bizarre moment in this cartoon.
Danny Phantom: While the second episode that showed a dragon (transformable via a magic pendent) played this trope straight, the second, more evil one, was much, much harder for the hero to defeat. In fact, his abused little sister was the one who ended it, but not before he created some serious damage.
The Batman Beyond episode "Splicers" has the villains mix animal DNA with their own. While the lead villain uses DNA from several different animals, he winds up as basically an eight-foot-tall snake (at least until Terry overloads him with injectors and he turns into a monster). He puts up a fairly decent fight against Batman (certainly better than he would have as a human), but still loses in the end.
Chase Young from Xiaolin Showdown often turns into an anthromorphic lizard when the going gets tough.
In the climax of TMNT: The Lost Episodes, the True Shredder demon transforms into a dragon to gain an advantage over the Turtles... At least, until the Turtles also transform into dragons. Because magic.
Samurai Jack: In Scotsman Saves Jack, the burly Scotsman tries to help an amnesiac Jack by retrieving his lost memories from a trio of sirens who hypnotize unlucky travelers. In the final battle, they combine into a fiery, three headed snake like creature and almost defeat the Scotsman before Jack finally remembers who he is and beheads (threeheads) the creature.
Dexter's Laboratory: One of the very few examples where it's a good character doing this. When DeDe was a baby she gave birth to an imaginary world called Koosland, with Peepers as her first creation, a cynical rabbit like creature who gave away so much happiness in the land that he left none for himself. One day, an evil animesque alien kidnapped him to use as an energy source. In the end foiled by the efforts of Dexter and DeDe's other imaginary friend, Koosy, the alien warlord meets his demise by being stomped by Peepers, who mutated himself in a flesh-colored, evil-looking dragon.
Transmetal IIMegatron. Considering all the bad guys who this doesn't work for, it seems ironic that he grows to hate his scaled up form despite it being responsible for his success.