In Abby Hatcher, Grumbles can shapeshift into almost anything. This even extends to other Fuzzlies as well.
In Adventure Time, Jake the Dog can alter just about every aspect of his physical being except for his coloring and in one episode he was even able to get around this limitation to pretend that part of himself was an entirely different person. One of his children, Jake Jr., has a lesser version of this power which is limited to her hair. Jake inherited this power from an alien shapeshifter who implanted an egg in Jake's dad Joshua's head, which resulted in Jake's birth.
D.U.F.U.S., Dr. Quark's robot from the Adventures of Sonic the Hedgehog episode "Robotnik's Rival", has this ability. At one point in the episode, he uses this ability to transform into Sonic and Tails and trick Robotnik and Quark when the real Sonic and Tails escape from Robotnik's dungeon. He feels guilty about tricking his master afterwards...but he gets over it, about ten seconds later.
The Amazing World of Gumball has Clayton and Penny. The former can take whatever form he wants because he's made out of clay. The latter's transformations are based on her emotions. Gumball himself can do this as well but this is mainly used for Rule of Funny.
The Tiger in “Aunt Tiger” can willingly shift from human to tiger form at ease and even disguises himself as the aunt at one point, though his tail still appears in his human form.
The Magician in Ewenn Congar can turn into whatever animal he chooses. So can Ewenn and then Inez, who use the magician’s magic against him in the end.
To a very limited extent, Meatwad from Aqua Teen Hunger Force. He can form a hotdog (made out of meat), or an igloo (made out of meat). Occasionally he takes other forms (meat bridge, Samurai Lincoln). And in The Movie an office park complex.
Inque from Batman Beyond has a body made of a black viscous fluid, and can shape herself at will.
Clayface from Batman: The Animated Series can assume human forms, and then change to a blob to attack if he is found out.
The titular Ben Tennyson can change into almost any alien using the Omnitrix and Ultimatrix. He can transform into aliens, originally choosing from a set of ten, with later additions. A future, 30-year-old incarnation of the character is stated to have 10 thousand aliens to choose from. It's taken much further in Ben 10: Alien Force. In addition to a completely different set of ten aliens from the original series, the season 2 finale has the Omnitrix's creator, Azmuth, activate the true Master Control function, allowing Ben to choose from 1,000,903 total alien species' from across the Milky Way galaxy.
The Lenopan species, which Ben's cousin Camille belongs to (her daughter Lucy is a human-Lenopan hybrid), is also capable of shapeshifting, although they don't inherit the special abilities (if any) of the species they mimic.
Best and Bester: The titular duo are unique in their neighbourhood, as they're the only ones in all Bettervale who can change their forms into something new. This allows them to perform numerous different tasks about town.
The 2006 revival of Biker Mice from Mars features Catatonian shape-shifters, who are Catatonians with the power to change their physical appearance. One notable member of them is Catalina Cat-A-Call, who tries to lure the Biker Mice into a trap by assuming the form of a Martian mouse and claiming to be Stoker's niece in "The Tender Mouse Trap" and later infiltrates the Martian military in an attempt to get to Carbine and impersonate her to deceive Throttle in the Clip Show episode "Carbine's Conundrum".
Klone from Blackstar is a rare example of a good-aligned shapeshifter, using his powers to combat the forces of evil and once to even sneak into the Big Bad's hideout.
Captain Planet and the Planeteers: In "Summit to Save Earth", the Big Bad Zarm pulls this off easily and perfectly multiple times, to the point of getting the voice just right. Most notably, he poses as the Earth Summit's president and manages to coax out the rings from the Planeteers by impersonating Gaia. The fact that he earlier has stolen the Heart Ring, thus making Ma-Ti unable to see through the ruse and alert the others until it was too late, certainly helped him in the latter case, though. Dr.Blight and Looten Plunder also put on helmet and transforms into the conference delegates through the machine
The Fleeblebroxians from the Chip N Dale Rescue Rangers episode "Dale Beside Himself" can shapeshift into whatever they want to. One of them spends half of the episode as a Dale Doppelgänger.
The Comic Strip: There's shapeshifting of the "aquatic" kind in Tiger Sharks. The main characters can turn into human-fish hybrids when they enter the machine known as the Fish Tank.
Courage from Courage the Cowardly Dog often assumes different forms when trying to explain to the Bagges that they're in danger or the person/monster/supernatural being/whatever they're dealing with can't be trusted, though it may just be done as a visual gag. However, there are times when Courage's shapeshifting proves to be very useful in tough situations.
The Flying Parallinis from the Cyberchase episode of the same name can change their shape by "flexing their angles".
Two episodes of Exo Squad feature a Neosapien who was transformed by Automutation Syndrome and medical experiments into a blobish creature that can shapeshift.
The Fairly OddParents! has mainly all of the fairies in fairyworld who can shapeshift at will due to their magic powers. Cosmo and Wanda can also do this — except in one episode where Cosmo's fagiggly gland acts up causing just the opposite to happen to him.
Nergal Jr from The Grim Adventures of Billy & Mandy effects this when he assumes teacher Miss Butterbean's form and gives the bully Sperg a good going over.
Goo from Gumby is the most clay-like of the Claymation characters therein.
Hazbin Hotel: Charlie Morningstar’s capable of transforming herself in a variety of ways to look more demonic such as growing horns, flashing razor-sharp teeth, turning her eyes red, and even growing a spiky tail. Given that her father has a similar form, it's implied she inherited this ability from him. Though, it is implied that she still doesn't realize just how powerful her shapeshifting abilities are.
In the Heckle and Jeckle cartoon "The Power of Thought", the two birds realize that they can change into anything they want simply by invoking cartoon physics.
The Herculoids: The title characters in the episode "The Mutoids" can assume the form of any humanoid or change themselves into solid rock.
The Martians in Invincible (2021), such as the Earth heroes Martian Man and Shapesmith, all have the power of shapeshift into pretty much anything. Their shapeshifting also grants them the secondary power to stretch their bodies like rubber. This is actually important In-Universe because Mars is currently infested with a parasitic alien species called the Sequids and the only thing preventing the Sequids from destroying the planet is their need to possess a native host to form a Hive Mind which is impossible because Martian shapeshifting makes them immune to being possessed. Despite their immunity, the Sequid threat is so great that the Martian Emperor ordered a quarantine of the planet and is heavily implied to have moved the entire Maritan civilization underground as a part of the quarantine.
Jade Armor: As a nine-tailed fox, Jiu Wei Hu can take on any form, from animal to human, that she wants. In her debut, she took on her original human form, the form of her former master's new henchman Will and that of a pigeon.
According to Word of God, Heloise from Jimmy Two-Shoes is "a bit of a shapeshifter". Exactly what this implies is vague, but it certainly isn't used very obviously on the show. "The Hooded Chicken" reveals that it's just her feet... whenever she has feet, that is.
Several aliens in Men in Black: The Series can alter their forms to conform to life amongst earthlings. An interesting case is a sociopathic alien who can willingly change forms, yet another member of his race is shown using a special suit like many other aliens in the show.
Middlemost Post: Parker can do this. Comes with being a cloud, supposedly. Makes sense, as clouds don't have much of a consistent shape.
Chameleon from Mighty Ducks: The Animated Series can shapeshift into nearly any sort of humanoid, even ones larger than himself. More importantly, he can disguise himself as a human to go where the other Saurians can't.
The Mighty Hercules features a beast called the Chameleon Creature, which can transform into anything it wants, even new forms it invents on the spot. An exiled Olympian named Otis gains its powers and becomes known as the Chameleon Man, and he can likewise turn into anything he wants, often disguising himself as harmless normal animals until transforming into a giant monster to attack.
In the Molly of Denali episode "Special Salmon", Raven demonstrates that he has the ability to take on an anthropomorphic (or perhaps fully human) form wearing a mask decorated in Northwest Coast design.
Goo from Monster Math Squad, being a Blob Monster, can shapeshift. In many instances where the gang does a plot-relevant countdown, he shapeshifts into a 3, then into a 2, then a 1.
Morph by Aardman, originally part of the Take Hart art show hosted by Tony Hart and then appearing in skits aired on Nickelodeon during the 1990s.
The luchadores from ¡Mucha Lucha!. At first, it seems like it's just to illustrate their attacks, but it turns out that they actually do this in real time.
My Little Pony: Escape From Catrina: Catrina's henchman Rep can change his shape on a whim, although he doesn't seem to be able to change his color scheme. Generally, he only briefly takes other shapes in order to deal with particular situations, such as by turning into a bird to rescue a falling bushwoolie or into a bull to charge down Catrina.
My Little Pony 'n Friends: Penna birds can change their shape at will. Over the course of "The Gost of Paradise Estate", Pluma takes on the forms of a ghost, a monstrous bat, a Giant Spider, a living mirror and a living prism.
Nightmare Moon can turn into a thorn, a cloud of starry mist, and even several ponies at once.
The changelings are a race that are capable of changing their form using innate magic. They are more-or-less pony-shaped in their natural state and are commonly seen copying the appearance of ponies — doing so convincingly enough to fool their friends. Their shapeshifting is more versatile than just that, however, and they can mimic other creatures such as dragons, and even inanimate objects like rocks.
In "It Ain't Easy Being Breezies", Twilight Sparkle demonstrates a new spell she found in the Castle of the Two Sisters that lets her transform herself and her friends into Breezies in order to help Seabreeze and his friends get home.
In "Twilight's Kingdom, Part 1", Discord once again demonstrates this power, both for the sake of puns, and to lure Tirek into a trap by turning into a unicorn stallion.
Newton, the big humanoid newt from Ned's Newt, can turn into pretty much anyone and anything (as long as he is fed with the special "Zippo" food; when he's not, he's just an ordinary boring newt). He is fond of shortly turning into a random celebrity to provide a joke, but often also uses this for his Zany Schemes.
The Oh Yeah! Cartoons short "Kameleon Kid" is about a boy who gains the power to change into animals and inanimate objects after a Freak Lab Accident.
Pac Man: Pinky of the Ghost Monsters can change his shape into anything he can think of.
Phantom 2040: Heisenberg is a collection of billions of microscopic fractal biots whose control module accidentally gained self-awareness. He can take any form, and even change colors at will, but his external "brain" is rigid and must be kept close (which usually means surrounded by his 'body').
Pocoyo: Pato is able to do this due to his detachable limbs.
Rainbow Butterfly Unicorn Kitty: Felicity (who normally appears as a horned cat with a rainbow color palette) can transform into four other forms (rainbow, butterfly, unicorn, kitty) with unique powersets.
Double Trouble's M.O. in She-Ra and the Princesses of Power. They treat shapeshifting as an art, completely dedicating themselves to becoming their target both physically and emotionally. They use their skills as a shapeshifter, The Social Expert, and a Master Actor to perfectly impersonate nearly everyone in the show, to devastating effect, before using their newly acquired knowledge of Catra's character to brutally Break Her by Talking.
Hordak, the main villain, uses cybernetics to transform either parts of or his whole body into different weapons and vehicles as he needed. Typically, his transformations are all metal, and some form of his face is present on the vehicle he turns into.
Similarly, his sidekick Imp can change shapes as well, utilizing his abilities to spy on the Great Rebellion. He also doesn't have the same limitations as his master, able to conceal his features, and even his color scheme in a chosen form. (Interestingly he can also manifest his face on a shape if he wants to, allowing him to communicate with others.) His only limitation seems to be he can only become something that approximate his physical size (i.e. he cannot become a giant T. rex or shrink down to be a lethal virus).
Shaggy & Scooby-Doo Get A Clue!: Scooby-Doo changes into whatever shape a situation calls for after eating the special Scooby Snacks that Shaggy's reclusive uncle Albert has been developing. Once it metabolizes and he burps, Scooby returns to normal.
Sofia the First: the eponymous princess gained the power of shapeshift thanks to the amulet of avalor into both, animals (birds, dragons) and magical creatures (mermaids, fairies) at will, normally using this power into her various protector missions.
In the South Park episode "Cancelled", the kids are captured by aliens. Their leader is a shapeshifter who initially takes the form of Stan's dad. The kids denounce this as stupid (a Take That! against Contact (1997)), and he goes on to take a variety of forms ripped from pop culture, all of which the kids reject. Finally, he asks them what they want — and they settle on "a taco... that craps ice cream", a form he assumes for the remainder of the episode.
SpongeBob SquarePants has the main character often transforming into a variety of different shapes and forms, such as in the episode "Gone", in which everyone in Bikini Bottom disappears and he takes on the forms of several characters to replicate interactions with them. Patrick also seems to occasionally transform.
Star Trek: The Animated Series: In "The Survivor", after the Enterprise crew rescues the philanthropist Carter Winston, he turns out to be a shapeshifting alien called a Vendorian.
In Steven Universe, all gems have shapeshifting powers, but the most common shapeshifter is Amethyst, who can shapeshift similar to Jake the Dog from Adventure Time, above — transform into anyone or anything, except her color.
Played with in an older Cartoon Network commercial. Zan and Jana are there to present to the child viewers the difference between real and imaginary scenarios, but the PSA is cut just short of revealing their "simple phrase" when Zan realizes (and begins complaining about) how he's always turning into something water-related, regardless of the situation at hand.
Supernormal: Changerella's power allows her to transform into just about anything she can think of.
Uni the Sea Urchin from Sushi Pack uses this ability which makes him one of the Sushi Pack's biggest threats.
Beast Boy from Teen Titans (2003), of course, can change himself into any animal that has ever lived, earth-based or alien — he just seems to need to know what it looks like. Being exposed to some chemicals in "The Beast Within" gave Beast Boy a powerful Werebeast form. While the transformation was involuntary in that episode, he shows the ability to transform into the Werebeast at will in "The End". Madame Rouge, one of the main villains from the fifth season, however, puts him to shame, being able to transform into anything, period, in addition to the ability to reform herself if killed. Heat's her only weakness, and once away from it she just shifts back into an undamaged form like with every other form of punishment that's been tried against her, to the point that even a character with magma powers was no match. She can also transform fast enough to stretch and grab Kid Flash. She's generally considered to be the deadliest one-on-one opponent in the show, barring the demon characters.
Worth noting is that the Transformers, at least, tend to choose new alt modes to blend in with new surroundings.
A special mention goes to Shockwave from Transformers: Animated, who has the ability to disguise himself/shape-change into another robot form, going under the alias 'Longarm Prime'.
David Bowie (yes, that David Bowie) has shapeshifting powers in The Venture Bros.'s world. In one scene he transforms into a bird and flies away, prompting Hank to comment that "the guy from Labyrinth turned into a bird and flew away!" A later episode reveals that he's not actually David Bowie, but is a big enough fan that he likes using David Bowie's appearance.
The Dragons in Wakfu are master shapeshifters, though all of their forms tend to share certain traits. Adamai for example favors a short humanoid form when not in his natural dragon form, but has also taken on the form of a little bird and a giant rock golem during combat.
Widget from Widget the World Watcher can shapeshift into pretty much anything as long as he isn't upside down.
The prof Wizgiz, as the professor of Metamorphosymbiosis, is able to shape shift his body into form he requires.
In the fourth season there's the shapeshifter Duman, one of the Wizards of the Black circle, who can turn into creatures, other people, and even water. Later on, his powers became unstable, which eventually kills him. Oh, and Darcy can also turn into other people with her illusion powers. Professor Wizgiz at Alfea is the transformation teacher, and he can turn into various creatures.
Yam Roll and his mentor Katcho Miso can both transform into a variety of objects, along with possessing a slew of other special powers.
The Zig & Sharko episode "Magical Jellyfish" features a jellyfish that can transform into any creature it sees; what set it apart from the originals is its aqua blue color and pink lips. At one point, Zig has it assume Marina's form so he can replace her with the jellyfish; because of the color issue he has to paint it to look like her.
