In fiction, when people turn into things, often times, the things will... look like themselves. Characters will often retain specific traits between physical forms. In some cases, this is as easy as hair and eye color, which makes sense to a degree. Things get a little confusing, though, when characters end up keeping a color scheme or markings based on, say, the clothes they were wearing.
It's done, of course, so the viewer/audience/reader can tell them apart from other animals or other people wearing the same outfit, if they're infiltrating and pretending to be someone else. It can also extend to the shapeshifter's voice, retaining their identifying vocal range, regional accent, and personal inflections regardless of whether their current body should be able to speak in that manner or not (sort of a non-malicious version of Voices Are Mental).
If these identifying cues become relevant to the plot, it will probably become a Glamour Failure.
Named after a not-so-popular hypothesis by biologist Rupert Sheldrake, in which the forms of organisms are imprinted upon "morphic fields", which in turn influence the forms of subsequent organisms (at least, before the term was hijacked by science fiction authors to name this trope's concept). May be brought upon by being a Baleful Polymorph. See also Shapeshifter Default Form, and Shapeshifting Excludes Clothing.
There was an episode with a Ditto owned by Duplica who had this problem — it could turn into anything, but it always kept its own face, which usually looked downright RIDICULOUS. It is eventually stolen by Team Rocket, who flat out threatened the poor thing, eventually teaching it to make a perfect transformation. Duplica returned in Johto with a second Ditto with yet another transformation issue — it could transform perfectly... just not its size when it came to big Mons. Duplica eventually decides that she'll make it her goal to try and capture an entire team of Ditto with various transformation quirks.
When Griffith incarnates as Femto in Berserk, one of his primary features is the demonic hood he wears, which is very similar to the helm that he wore as the leader of the Band of the Hawks.
In Princess Tutu, Ahiru's duck form has a sometimes disturbingly human face and retains her trademark hair-style. Or maybe it would be more accurate to say that her human form retains the trademark feather-style of her duck form.
In Ranma ½, most of the transforming characters retain some of their regular characteristics.
Some are minor and make sense, like Ranma retaining his hairstyle because there's nothing in the transformation that would require undoing his braid.
Genma's panda form has round black circles around the eyes, looking very much like the glasses he never part with in human form.
Shampoo's cat-form markings match the clothes she wears most of the time, and her fur is an unusual shade of pink and purple in the anime. She also always keeps the bell-like, round hair decs tied to her bangs.
Mousse keeps his fringe of hair as a duck, as well as his Opaque Nerd Glasses. The glasses don't shrink, however — he owns a second pair sized for his duck form (this can be seen clearly in the manga, book 10 chapter 5).
An Imagine Spot where the characters envision Happōsai changing into a tiger, a monkey, or a snake, has them all imagining animals that clearly have Happosai's features.
Rakkyōsai's tiny ponytail and swirly spiral markings on his cheeks remain in place whether he's a child or a tiny old man.
A monkey thrown into Nyannichuan, which had a very unique and nearly-human look compared to the rest of Takahashi's monkeys, retains her basic facial appearance as a human.
Weirdly, Rouge's translucent pants disappear whenever she becomes Asura and come back when she reverts, even though her lower body (and the rest of her wardrobe) doesn't change at all. Her waist-length hair also disappears, though that may simply be due to how hard it would be to draw long hair for one head with three faces.
This has also caused no end of controversy as to what the extent of curses' transformation really is. The common consensus is that cursed characters transform into "what they would look like" in the alternate form, and this is proven right by the Musk Dynasty saga (wherein another Gender Bender victim of Nyannichuan looks nothing like female Ranma, but like a female version of himself, and various animals thrown into the spring also have unique appearances and the similarity between two Nyannichuan victims is described as so rare it's actually a plot point), but muddled by the Phoenix saga (where the Phoenix people "somehow" manage to create a spring which does turn you into an exact duplicate of the original).
Naraku, much to his frustration, retains the spider-shaped burn scar that marks him as part-human. The latter is so persistent that ripping the skin from his back or burning it off (which his Healing Factor enables him to do without any long-term consequences) will only result in his skin growing back with the mark intact. The same applies to all of his detachments.
Averted in Wolf's Rain. It would have made perfect sense for the wolves' fur color to match the hair of their human forms, but the colors are reversed for Kiba. Having black hair in his human form actually makes a lot of sense — it makes his skin and eyes stand out a lot, so it's not too much of a stretch that the guy with light eyes and pale skin is really a white wolf.
As witch-apparentices, the main characters of Ojamajo Doremi could turn into pretty much any animal they wanted. However, no matter what form they assumed, their fur would always be of the girl's Signature Color and Doremi would always end up with Odango somehow, even if the animal in question didn't even have hair. Likewise, Hazuki somehow never lost her glasses upon transforming.
There are several instances of this in Hell Teacher Nube, especially when it comes to shapeshifters:
An accident with a magical artifact temporarily shoves Miss Minako's soul into Miss Ritsuko's body, resulting in a mishmash that calls herself "Miss Rinako". Her appearance is a combination of both women right down to the outfit and hairstyle.
Whenever Nube is completely taken over by the Oni Hand, he looks nothing like Baki, the actual Oni sealed in his hand, but as Nube himself with monstrous features.
Hayame the mermaid, who has several distinguishing features in her true form (such as webbed hands, large gills on her ribcage, and fin-like ears,) keeps her exact same face and hairstyle as a human.
Subverted with Tamamo, the yohko. This type of kitsune steals human skulls for his own use, which grants him the exact appearance of whoever the previous owner was (up to and including hairstyle.) This causes some discomfort when the fiancée of his last victim shows up, thinking that her missing fiancée has finally come back from his hiking accident.
A mystic tree in the school's playground releases insects that have a heavy resemblance to whoever sticks their finger in a knot and gives it some blood. As in, when the girls tried it, they got butterflies and moths that looked exactly like them but with wings. However, these insects quickly degenerated into horrifying "halfway" states. Nube settled the commotion down by producing Nube-faced Hercules beetles complete with gloved Red Right Hands that subdued the monstrous bugs.
The spirit of bedridden Ill Girl Ayumi, who attends school via astral projection, still wears her glasses even though a projection wouldn't need them. She clearly does this to preserve her self-image, since she, Tamamo, and Nube (who taught her how to project) can modify their projections at will, to the point they could be blobs of ectoplasm if they wanted.
The Oni Siblings are all over the place: Baki's human form is heavily muscled, has wild hair, and prefers intimidating clothes like biker jackets and chains, and he closely resembles his original shape. Zekki's human form is a wispy bishonen that is nothing like the huge and horrific monstrosity of his Oni appearance. Minki looks exactly the same whether human or Oni, just minus the horn and the monstrous right hand from the latter form.
Partly averted in Yaiba, where the some of his transformations made with the Gold Orb are complete, but played straight by Shiro Amakusa who retains his whiskers and sometimes his collar while transforming.
007 of Cyborg 009 can turn himself into anyone, but he always retains his decided pronounced belly button, and reverts to his original form if it is pressed.
In Transformers Cybertron, as per usual the Transformers' keep their existing colors when they scan new modes. Additionally, their weapons and their Cyber Key slots and components impose themselves on the new form.
Attack on Titan has this in rather brilliant foreshadowing for the identities of the Titan Shifters - The Rogue Titan shares Eren's hair and eye color, the Female Titan is essentially what Annie would look like skinless and 15 feet tall, and the Armored Titan is built similarly to Reiner and even shares his "hairstyle". This is less pronounced for Bertolt (the only similarities seem to be the height) and Ymir (whose Titan form hardly resembles her), but there are still some similarities.
A horrifying example occurs when Connie holds up a picture of his mother in front of the deformed Titan in the ruins of his house, emphasizing how much the Titan resembles Mrs. Springer and the sinister origins of the titans.
One of the Pokémon TCG expansions included cards of Ditto mimicking various Pokémon:they all have in common Ditto's wide mouth and dotty eyes.
(Almost) every Magic: The Gathering creature card with the Morph ability has a "morph spider", either the creature itself or in the form of a subtler object, hidden in its artwork.
In One Disney's Aladdin Comic, the genie switches Aladdin and Jasmine's bodies to teach them a lesson, and their clothes change colors to reflect their normal colors. (Aladdin as Jasmine's top and pants are purple and white, and Jasmine as Aladdin's vest and pants are blue.)
Plastic Man could become almost anything, but kept his colors. Whether or not he has to keep his goggles and other facial features depends on the writer and/or medium. His power is actually being semi-fluid and he is therefore able to stretch/contort into the shape of anything. He does not actually turn into the thing. There is a JLA comic where Plastic Man turned into an aeroplane that could actually fly. And, with tremendous effort, he once turned the tip of his nose blue.
The Impossible Man character in the Marvel Universe and all the other Popuppians. He can shapechange into different forms but always keeps his green and purple coloration. This is, in fact, how he lost a Shapeshifter Showdown once. After trying to impress each other with a wide variety of assumed shapes for the entire issue, his opponent Warlock (with a little help from his teammates, who felt the whole thing was getting out of hand) hit upon the idea of changing color, which he could do and Impy couldn't.
Warlock at one time was only able to shapeshift into black things with yellow circuitry. But he got better at shapeshifting and that quality went away.
Obscure but awesome Marvel character Dirtnap's forms always kept the smiley-face (classic yellow-circle smiley thing) on the stomach.
Changeling (a.k.a. Beast Boy) from the Teen Titans is always green in his various animal forms.
As of New 52 he's always red now, as they've tied him into The Red (the metaphysical manifestation of the Animal Kingdom which Animal Man draws power from, and the animal equivalent to Swamp Thing's The Green).
As is Virginia Dare in Marvel 1602 (which is loosely based on the legend that she could turn into a white deer).
In X-Nation 2099, the mutant Willow always has a particular marking around her left eye, even though her shapeshifting powers are otherwise so precise that she can duplicate another person's form right down to their DNA.
In Avengers: The Initiative, secret Skrull spy Crusader has a flashback to a combat training exercise on his homeworld where his opponent K'rtem gave him a scar. Crusader vowed to keep the scar no matter what form he took in tribute to K'rtem's might.
When Maleficent from Sleeping Beautyturns into a dragon, she retains the black-and-purple colour scheme of her wardrobe. Even if you hadn't seen the movie and were just shown stills of Maleficent as a fairy and as a dragon, it wouldn't be hard to tell they're the same character.
When Kenai is transformed into a bear at the very beginning of Brother Bear, if you look very closely, you can actually tell that his bear form has the exact same facial features as his original human form. Also, his fur is the same color as his clothing.
When Mater disguises himself as a Lemon tow truck in Cars 2, he actually retains his hazel eyes (and yes, the opposite is true). And his body style when he decides to have fun with his disguises, such as a German SUV wearing lederhosen, a Dracula-esque hearse, a yellow drag racer, and a taco truck.
And Rod "Torque" Redline's Gremlin disguise. Since his disguise is made up of a retractable metal shell instead of a voice-activated hologram projection, the only thing about him that doesn't change are his hubcaps.
Aisling from The Secret of Kells is a white-haired girl with green eyes. This is reflected through the animals she can shapeshift into (mainly a wolf, but the first few minutes of the film show that she can become a salmon and a deer as well) since they will always have white colouring and green eyes to show that it's her.
Films — Live-Action
In the X-Men movies, Mystique's eyes occasionally flash yellow when she's shapeshifted into someone else as an indication it's her. She also keeps the scars Wolverine gave her in the first movie no matter what form she takes.
On the Discworld in general, nothing, not even a god, can change the nature of his or her (or its) eyes.
In The Last Continent, the Librarian gets a magical illness that makes him turn into all kinds of things, all of which have shaggy red fur. This includes a book, various pieces of furniture, and a dolphin.
In the TV adaption of The Colour of Magic, the Librarian's human form has a thick auburn beard. He really does look like he's waiting to turn into an orang-utan.
And Angua, a werewolf with long blond hair, turns into a wolf with long blond fur.
The Animagi in Harry Potter, particularly McGonagall and Rita Skeeter. Their animal forms both have their "glasses", in that the characters "wear" glasses-like patterns on their animal faces. This is at first hinted to be deliberate, as Animagi have to register and have distinguishing marks, but then Rita's an unregistered Animagus and she has such markings, so it's apparently intrinsic to the process.
This even extends into their personality in some cases (though it's a "chicken-and-egg" question as to whether their animal transformation affects their personality or the other way around). Sirius is often described as having "a bark-like laugh" and shaggy black hair. He transforms into a large, shaggy black dog.
Heckedy Peg, a children's picture book by Audrey and Don Wood, features a unique take on this that's critical to the plot: A witch turns seven children into seven different inanimate food items, and their mother has to guess which child is which in order to rescue them. While each child's food form is otherwise indiscernible from real food, it turns out that they had each turned into a type of food that matched the present they had asked their mother to buy for them earlier in the story (The child who turned into bread had asked for butter, the child who turned into cheese had asked for crackers, etc.).
In Lirael, when Lirael is exploring the Great Library, she uses various shapeshifting spells to turn into animals. For one of her later explorations, she's using an otter-form and notices that the fur seems darker than normal. As otters are colourblind, it's not until her friend, the Disreputable Dog, tells her that "otters shouldn't have red bellies" that Lirael realises that she has subconsciously changed the fur to match her promotion within the Library to a red waistcoat.
In Holly Black's Modern Tales of Faerie, faeries keep a trait when they transform. The troll Ravus, for example, turns into a human with golden eyes.
Journey to the West: Sun Wukong, the Stone Monkey, can use Taoist arts to perfectly change into many different animals — but if he tries to become a human, he can only change his head and must conceal everything else.
In the German novel Krabat the Master, the Evil Sorcerer can turn into different Animals, but they will always have only a left eye. The same counts for his students, whose Animal forms always mirror their appearances as humans.
In Beastmaster, King Zadd is turned into a donkey with the same tattoos. Although one character finds them familiar, it doesn't become Glamour Failure in time to be meaningful.
Since Kamen Rider Decade, who has the power to transform into his predecessors, needs to access the DecaDriver to use his own abilities, the Driver appears on all his forms.
Myths & Religion
According to tradition, Satan can transform into any creature, every part of him — other than his feet (either cloven hooves or chicken feet). Fridge Brilliance: This must be why he prefers snakes. Nothing would give him away in that form.
The alp, a nightmare-inducing, blood(and breast milk)-sucking creature from German folklore, derives its shapeshifting abilities from its magic hat, which it always wears, and which is always visible in some form no matter what it shapeshifts into.
In Japanese folklore, kitsune (fox spirits) can shapeshift but will retain their tails (like the examples from InuYasha and Dungeons & Dragons). Therefore, they must cover up their tails in a more conventional manner to hide their true nature.
On the Roger Miller episode of The Muppet Show, an outbreak of cluckitis, a disease which nearly-instantaneously turns the afflicted into chickens after they sneeze, sweeps the cast. The Swedish Chef retains his mustache and eye-covering eyebrows. Lew Zealand, Rowlf, Miss Piggy, and Janice keep their respective eyes; Kermit does, as well, and even has his pointy collar. Only the main cast retains Morphic Resonance, however, and some don't even get that: extras and even some of the main cast turn into indistinguishable chickens. Maybe they'll have a hat or necklace, which they keep.
The hengeyokai race in the 1st Edition supplement Oriental Adventures. When they shapechange into human form, they always have a distinctive feature from their animal form, such as a sparrow hengeyokai with a long nose like a bird's beak.
Dungeon Magazine #21, adventure "The Chest of the Aloeids". The goddess Athena is noted as always having grey eyes, no matter what form she assumes.
Lunars in Exalted each have a unique body feature called a Tell, which is retained regardless of what they transform into. Their magic makes their Tell difficult for normal people to notice, but once you do, you'll always be able to identify that Lunar at a glance, regardless of form.
One of the sample archetypes in Mutants & Masterminds is an Indian woman with a blue costume and a bindi dot on her forehead. She transforms into blue animals with little red dots on their foreheads.
When Chaos Lords and Sorcerers of Warhammer 40,000 transform into Daemon Princes, there's a good chance that they'll still be clad in the remnants of their old power armor. That being said, not much else is recognizable...
BIONICLE: Word of God says that Kanohi Masks generally retain their basic shape if their wearers willingly shapeshift, but there's not much in the merchandise and media to suggest this.
Pretty much any video game where you turn into something beyond just a standard power up.
Druids: Elves keep their long eyebrows and Pointy Ears (even if this makes their bird forms rather silly-looking), and Tauren keep their horns. In Cataclysm, with the addition of Troll and Worgen druids, Trolls keep their tusks and mohawks, while Worgen's forms have glowing eyes and manes.
Worgenin general play this trope straight. Every aspect of a worgen's human form has an equivalent in worgen form. For example, darker skin in human form results in darker fur in worgen form; braided hair in human form equals braided hair in worgen form, and so on.
Also, Alexstrasza still has her horns, glowing eyes, and necklace in humanoid form as in her dragon form, along with a wing-like cape. Her sister Ysera gets similar treatment. Black dragons also always have humanoid form as humans or elves with black hair.
In the first game, Donald Duck and Goofy change into a half-octopus (cecaelia) and sea-turtle respectively in the Atlantica level, though they still have their distinctive faces.
In Kingdom Hearts II, in addition to the Atlantica level, there's also the Pride Lands level. Donald is a bird, and Goofy is a tortoise, both with their own faces. Sora is also transformed for the Pride Lands level; he's a lion cub, with his blue eyes, crown pendant, and distinctive Anime Hair.
Furthermore, abuse of his Super Mode can randomly transform Sora into a Heartless version of himself, completely black and with glowing eyes. Which is funny because he actually became a Heartless in the first game, and he looked like a small basic enemy indistinguishable from any other Heartless.
In Scaler, after being turned into a lizard, Scaler retains the blue-and-yellow colour scheme of his clothes, and also a sort of scaly, spiky version of his blonde hair. Likewise, when transformed into other types of lizard-y creatures, Scaler still keeps a blue-and-yellow colour scheme.
You can always anticipate who "Louis Cypher" will be because he (or she, as the case may be) will always have intensely golden hair and blue eyes, regardless of the setting of the game. And in all but one case, he will also have long hair. Especially notable in Shin Megami Tensei III: Nocturne, where he assumes two different forms to mess with you.
In Shin Megami Tensei: Strange Journey, after Jimenez fuses with Bugaboo, the resulting half-demon looks exactly like half-Jimenez, half Bugaboo, with half of the latter's face acting as a mask to cover Jimenez'. This is a very unique transformation because fused demons always result in a completely different kind of demon.
In Persona 3, the Ultimate Persona of the Death Arcana, Thanatos, wears a mask that is like a highly stylized version of the Death Shadow's head. Additionally, Orpheus, the main character's initial Persona, has his or her same face, and when Thanatos and Orpheus combine, the resulting Messiah shares traits from both.
In Persona 4, most of the Personas earned by the protagonists look like the Shadows they originally were, with minor differences to make them less nightmarish and more heroic. Amusingly, although Take-Mikazuchi looks little like Shadow Kanji, he DOES have the skull-and-bones-on-black motif that Kanji's clothes have.
In Nocturne, Noah, the demonic sponsor for Isamu's Reason, eventually grows a face like Isamu's. And his HAT.
The Strange Journey example above parallels the Chaos Hero of the first Shin Megami Tensei game, who also fuses with a demon yet remains looking mostly human but with some demonic features. By the same token, Zelenin of the later game is a parallel to the Law Hero of SMT1, who is transformed by angels to become the Messiah in a world of God's order and stagnation.
In Version 6 onward of Ao Oni, your friends will transform into an Oni if they are killed... but retain their human hairstyle.
In Professor Layton And The Miracle Mask, one of the earlier "miracles" by the Masked Gentleman had him turning random townspeople into horses, with the victims retaining a few items of clothing. In reality, this was just an invoked trope. The Masked Gentlemen had accomplices hidden in the crowd, who (whilst the crowd were distracted) slipped away, replaced by horses (wearing the same clothing items) which were sent out from their hiding places in alleyways, tricking people into thinking that a transformation had occured.
At least in early generations, any Ditto who used Transform would become a perfect copy of the enemy sprite, but would retain its purple palette.
As in the anime, the Ditto in Pokémon Snap also have a problem with their faces not changing. In fact, it's the only way to tell them from normal Bulbasaur (which they will register as) until you hit them with a Pester Ball to make them change back.
In Plants vs. Zombies, the Imitater will turn into any non-upgrade plant, but will stay in grayscale after transformation.
When the dragon in Dra Koi takes human form, her eyes stay slitted and reptilian and she keeps her fangs, though mostly in cute form.
When Maya or Pearl channels someone in the games, they change their size and faces to the channeled, but they retain their hair style, color, and their clothes. This is arguably because hair and clothes are "dead" and can't be affected by living tissue. Their full bodies change completely into the channeled person.
A strange, non-supernatural example: when de Killer testifies in court in the second game, the transceiver he's using to communicate with the court has a button resembling his monocle.
Whenever Myan transforms in Cat Nine, she gets to keep her magic collar, her facial markings and her eye and hair color.
In El Goonish Shive, the artist Dan Shive often draws characters transformed into animals or anthropomorphic shapes (sometimes pertaining to the storyline and sometimes not), but they always retain their original hairstyles. Often, the fur will be the same color as their hair. As for Grace, well, her use of this trope is apparently down to the strange synergy of her mixed Uryuom and Lespuko genes, which enable her to instinctively blend the most desirable features of all her available forms — such as maintaining the expressiveness and speech-ability of a human face while in an animal-form.
Evilish features a character that can change into almost anything, but will always keep triangular facial markings under his eyes.
In Gunnerkrigg Court, when the Kershaws transform into birds, the feathers on their heads mimic the hairstyles they had as humans.
In The Inexplicable Adventures of Bob!, Princess Voluptua's alien butterfly form retains her long red hair and has roughly the same silhouette as her human mask (except for the wings, of course).
Last Res0rt has this enforced on Alice, sort of; she can't change the color of her eyes. This would be more of an issue if her eyes hadn't already changed into the same red-pupils-in-black "Dead Eyes" that every Djinn seems to have, but among the limited cast, it's trouble enough.
Each of the Mons in But I'm a Cat Person has a few visual traits that stay consistent across all human and animal forms, if only for the convenience of the reader. Cybele's pink eyes, Patrick's brown-and-blond hair/fur, Reseda's sharp teeth and claws, and so on.
In Runewriters, whenever Sev shapeshifts his new form tends to have greenish skin or fur to match his hair.
Nimona: Nimona the shapeshifter is a redhead, and thus about all her alternate forms have red or pink tones. Ulike many examples of this trope, she actually plays with it once by using a rat painted pink as a diversion.
Fauna of DC Nation maintains her blonde hair, bipedal form, and cannot change her height.
Some of the Trainers in We Are All Pokémon Trainers tend to retain some physical traits when they transform into Pokémon. DS, for example, keeps her glasses as an Eevee, then her green eyes when evolves into an Espeon. Tagg keeps a mole under his left gill when he turns into a Mudkip (later Swampert).
Similarly, Diane — who had spent most of her life as a Staraptor — retains qualities such as her black-and-red hair resembling a Staraptor's crest.
In Vaguely Recalling Jo Jo, Oingo always keeps a shirt labelled Oingo no matter what form he takes, which gets him spotted by Polnareff
In Aqua Teen Hunger Force, a running joke is that Meatwad can change shape, but only into either an igloo, or a hot dog. Both of these still have the same texture and color as Meatwad, and still have his face. In earlier episodes, he could change into other things, such as a Meat Bridge and a statue of Abe Lincoln with a Samurai sword (it was supposed to be a statue of Wayne Gretzky). He became a hammer in a much later episode, too. In The Movie, he turns into a full-size office building and a giant monster designed to fight a rampaging exercise machine. It makes sense in context. Sort of. He retains the previous clues to his identity, the texture and face and whatnot. Justified since Meatwad isn't a true shapeshifter. He's just a ground meat patty than can change his shape.
Each of Ben's alien forms has a "timer" resembling the face of the Omnitrix, in addition to, for most of them, an outfit design similar to that on Ben's one T-shirt. At least one of them has the design (a black stripe on a background of white) on its skin.
In "Gwen 10", the colours become light and medium blue, reflecting her one T-shirt.
Thesequels don't have the clothing match-up, though the Omnitrix symbol remains. It turns out that this is the Omnitrix itself, which changes size and shape to fit its user in any form. The fact that it has a built-in limiter and Ben hasn't mastered it is why he can't change at will from one form to another just by touching the omnitrix in any form. (Also, it's for this reason that it's supposed to always appear in easy reach on the chest as in AF and UA, not in a random place as seen in the original series.
Acknowledged in the Ben 10: Omniverse episode "Ben Again". Younger and older Ben team up in the climax, and when both turn into one of older Ben's standards, younger Ben retains the white shirt and black stripe.
In Gargoyles, no matter what form the Weird Sisters take, they always have their respective hair colors.
When Transformers reconfigure themselves to have different vehicle forms, their robot-mode bodies tend to look roughly similar to their previous forms.
For example, Generation 2 gave Optimus Prime a new "Hero" body◊ that still had his distinctive red truck cab chest, even though his chest actually became the underside of his new vehicle mode (which had a white cab of a completely different design).
Mind you, that's a major improvement over the original series, which applied the same concept going backwards in time. G1 Bumblebee had VW Beetle hood halves on his feet long before he turned into one (indeed, long before there was any such thing as a Beetle, or Volkswagen, or Germany, or humans at all).
This is something in several series (but not in just as many others). Rather than make two designs for every character, a pre-Earth one that will only be in part of the premiere as well as their standard form they'll have the rest of the series, the standard robot modes will be used in Cybertron scenes and they just won't transform (except in G1, where alien vehicles transform into robots and 'grow' their eventual Earthen vehicle parts on the way.) Transformers Animated seems to be the only series so far that gives everyone pre-Earth and Earth forms in vehicle and robot mode (they've all got toys, natch. At least, the main Autobots do.) They actually get quite a bit of mileage out of the pre-earth forms: characters still on Cybertron will be the Cybertronian forms repainted; apparently, there are only so many body-types a 'bot can start out with, and it's through scanning new ones on their own later that more unique forms are obtained.
Transformers Animated did a similar plot in the two-part "Human Error". This time, the resonance was justified by the world where the Autobots were human being a simulation run by Soundwave, who was brainwashing their bodies to fight for him while their minds were preoccupied.
This happens in one of the later episodes of The Magic School Bus, where the kids learn about animals living in the city, and their hairstyles are kept when they're changed into animals. The same could be said for any of their other transformations, including those The Bus undergoes.
In season 2 episode 5, Yugo, Amalia, and Evangelyne are turned into piglets, but retain some recognizeable traits, like Eva's green eyes and freckles, or dark skin and tufts of green hair for Amalia.
Also happens with the heroes changed into Thursters in season 2 episode 13. You can still recognize them, which make the scene where Evangelyne and Sadlygrove try to touch each other that more pathetic.
And again in season 2 episode 21, when the Bellaphones turns the male members of the party into apes or monkeys.
Here's a tip for new viewers of Samurai Jack: if Jack encounters a character or creature with red, green, and black predominantly featured in its color scheme, it's going to be Aku in disguise. Yes, this even applies when said character is the Mad Scientist's Beautiful Daughter.
Although he's not a shapeshifter, Danny Phantom's green eyes show in the face of whoever he's overshadowing. And that's true for other ghosts as well. When he turns into his ghost form, the suit he wears is the same one he wore when he first transformed into a ghost.
Strangely enough, on Superfriends the Wonder Twins have differing shapeshifter tells: it was Zan whose face was usually visible as a reflection on the surface of whatever water-based item he turned into. Jayna maintained her purple eyes no matter the form she took.
All fairies from The Fairly OddParents maintain their primary characteristic when they change into something else. Their crowns frequently remain above their heads no matter their shape as well. This is also true for other nonhumans.
Cosmo's and Wanda's hair colors are their primary trait, so they turn into green and pink items. Wanda and Cosmo's unlikely colored forms are often Lampshadeed by anyone with enough attention to notice that dogs shouldn't have pink or green fur. This went to Serial Escalation levels when they agreed to switch roles with Timmy and transforming themselves into looking like him — except their hair, voice, and eye color remained the same as their default forms. This became a source of confusion to Timmy's dim parents whenever they encountered one of them breaking house rules.
A Couch Gag parodying human evolution actually showed various prehistoric animals resembling Homer Simpson gradually turning into, well, guess...
There is also an octopus resembling Mr. Burns, a dragonfly resembling Lenny Leonards, a Pterodactyl resembling Agnes Skinner, a Tyrannosaurus rex resembling Bart, and a Stegosaurus resembling Lisa.
And a small mammal resembling Moe Szyslak. (Which did evolve from Cave-Moe.) Or Cave-Moe devolved back into a rodent, to keep with the theme of his occasional attempts at suicide.
A "Treehouse of Horror" episode was actually about Dr. Hibbert being a mad scientist who turned everyone else into an animal who retained characteristics of who they originally were: for example, we have Homer as a walrus, Marge as a jaguar, Bart as a spider, Lisa as an owl, and Maggie as an anteater.
The mascaritas' Signature Moves from ˇMucha Lucha!. For example, Ricochet's "Pulverizing Pinball" has him turn into a large sphere colored to resemble his wrestling mask, and Buena Girl's "Bulldozer of Truth" has her turn into a bulldozer with her outfit colors.
Yam Roll of the so-named cartoon can transform into pretty much anything, but that anything will be coloured like his yam filling and wearing his trademark cowboy hat. His mentor has similar powers, always turning into something brown with a white beard.
Tom Terrific, the 1950s tv cartoon character, could transform himself into anything — but always kept his face and trademark funnel hat.