Animation as a whole slowly went through this phase from The Silent Age of Animation to The Golden Age of Animation. Early cartoons were very crudely made—they were very stiff, rigid and mechanical in appearance and movement, had no construction, no line of action, lots of symmetry (which made them look flat) and body parts piled onto each other, rather than being directly connected by form. This began changing when Walt Disney began forming and refining The Twelve Principles of Animation, as well as animators like Fred Moore altering Mickey Mouse's design to become more pearlike and organic, allowing it not only to be three-dimensional, but also to be more pliable and organic than the earlier, rigid designs from shorts like Plane Crazy and Steamboat Willie. Disney immediately adapted this to its other characters, and everyone else in the animation industry copied this immediately, sending classic rubberhose animation to its grave within a few years.
The kinds of mediums used throughout the history of Western Animation evolved. During the silent era, animated productions utilized a sketchbook aesthetic, consisting of solid black on white for both characters and environments. Then, as the Golden Age set in, the art became more complex, with each cel being specifically shaded different kinds of gray to appear similar to a live-action black and white movie. The shift to color occurred in two steps: the adoption of two-strip Technicolor & Cinecolor and the adoption of three-strip Technicolor, with the former bringing visuals that made clever use of varying shades of red and green, and the latter resulting in bright, saturated palettes that resulted from the dye-transfer process used by three-strip Technicolor stock. Later, the switch to limited animation in the Dark Age brought on extremely simple color palettes that would save both time and money, a change that would gradually reverse from 1987 onwards thanks to the onset of the Renaissance age. When the medium switched to digital in the late 1990s, palettes became significantly louder and lines & shapes became conspicuously sharper. For reference, compare thesetwo screenshots from SpongeBob SquarePants (cel on the left, digital on the right).
Batman: The Animated Series had very full and weighty character designs and animation. When it was Un-Canceled, the designers wanted to modify a lot of the designs for several reasons: the weightier designs were more difficult to animate, improve design "mistakes" in their opinion, simpler designs made the emotional actions clearer. Many fans didn't like the relatively "blander" designs, especially the new Joker. (Even the creators acknowledge it didn't turn out the way they wanted — it looked good on paper but not animated, and lacked the deep redness of the lips that gives him such a manic grin). This said, some, like Scarecrow's new nightmarish appearance, Poison Ivy's green skin, and Penguin's less Burton-esque revamp came out okay. A good discussion of the changes can be found here.
For purposes of more illustration, take a look. The same villain (Baby-Doll) as seen in Batman: TAS and TN◊BA.
Two-Face was one of the few to remain virtually unchanged. His new look was crisper and more in-line with the rest of the DC Animated Universe (having been given more squared-off shoulders and sharper lines on his suit), and the damaged side of his face was changed from dark blue to turquoise, but other than that, he remained virtually the same.
With Batman Beyond and more so in Justice League, the Timmverse got its final revamp. While the Batman: TAS era had every character given a weighty look and the Superman: TAS/TNBA era gave everyone a sleek look, the 3rd revamp added more lines. The best example of the changes would be the Joker, though, as he was present in all three eras and was wearing the same clothes, for the most part. In the Batman: TAS era he had a rounded face and detailed features. In the Superman: TAS/TNBA era, he was drawn very basically with few lines. Even his eyes were simplified and his mouth became simple curves. In the JL era, he got more lines and a face more similar to his Batman: TAS design, but with the sharper lines and details (and red irises in his eyes.)
Batman is a good guide as well. Here are three images, from Batman: TAS, The New Batman Adventures, and Justice League (Unlimited). Notice how the inner leg in his TNBA design is represented by a single curved line, while in his Justice League design there is a distinction between thigh and calf, and in his Batman: TAS design both the thigh and calf have curved lines. And the gauntlets are different. In his TNBA design, they are drawn in a single zigzag, and in his Justice League design one each is its own defined curve.
When the Green Lantern Corps first appeared in Superman: The Animated Series, all of its members wore identical uniforms. This continued for the first two seasons of Justice League, with John Stewart wearing more or less the same uniform sported by Kyle Rayner and the other GLs back in Superman: TAS. Then, starting in Justice League Unlimited, the Green Lanterns were suddenly shown wearing individual costumes that differed greatly from character to character, with Kyle now sporting a new outfit similar to the one he was wearing in the comics at the time.
Abby Hatcher has much smoother and more shaded animation since Season 2.
The animation and character designs in Adventure Time have gone through some pretty noticeable changes over time. Finn, as a result of going through puberty, is noticeably taller, Jake's design is not as circle-like as it used to be and is a bit more detailed/streamlined. Marceline is no longer a total noodle person, as she has gained some curves over the course of the show (compare "Evicted" from season 1 to "Red Starved" in season 5 and one can actually make out hips and breasts on her design in the latter). The animation itself has also gotten smoother and less overtly cartoony over time, and background details are more rich in the show's later seasons.
The second season of American Dragon: Jake Long animation changed to a more detailed and fluid style of animation compared to the first. The character designs changed considerably. For example, Jake's dragon form was muscular in the first season and sported wings but in season two and onwards he is scrawny and is styled after Asian dragons. The Oracle twins also switched from identical redheads to a black haired raven and blonde prep.
The series was never very solid to begin with, but it somehow improved vastly. Initially, Meatwad's shape would vary constantly, before settling in on its more meaty roundness at some point in the middle of the first season. The biggest evolution was the vast improvement in lip syncing, where the first episodes typically had the characters' mouths move in the same one or two animations for all dialogue, they now move faster and more accurately to the words. Later episodes also saw animation breakthroughs such as Carl running (complete with jiggling!) and loads more realistic animation and poses for real people and the Aqua Teens.
The serialized Teens are quite different in appearance (especially Frylock), and distribution of personality, to their original appearance on Space Ghost Coast to Coast.
The Backyardigans' five-kid-cast got improved with Season 2. You can barely notice any change on Uniqua, and it's more noticeable on Austin (who looks nothing like he did during the first season).
Beavis and Butt-Head had developed its own refined low-key style of animation by the last few seasons. The very early episodes comparatively look like they were drawn by blind toddlers. In fact, creator Mike Judge hated the first season because of how bad the animation was. The revival makes the shift more obvious when comparing their characters' designs to the rest of the universes.
The Ben 10 franchise had changed in looks between the original series and its sequels to look less cartoonish. For example, Ultimate Alien, which brought back the 10-year-old Ben in one episode, had slight changes in detail to him and the aliens he used. Zig-zagged, now they have shifted 3 times. Omniverse's artstyle is back to being cartoonish, but utilizes much more simple character designs and colors, creating a much more colorful and fluid show.
Big City Greens sports smoother, more detailed animation post-2019, such as the pupils of Cricket, Tilly and Gramma appearing not as big as they did in the early episodes, and Officer Keys' mustache becoming longer and curved. Also, the emotions are more fluid, and even the title cards have improved.
Blaze and the Monster Machines had a major design and animation change partially into its third season. The backgrounds become vibrant and more detailed, such as visible asphalt on the streets, better lighting and shading on the buildings, and the trees are different. The vehicles are now animated with ray-tracing, and their tires have visible lead. The animals are also radically redesigned from their original cartoonish appearance to a more realistic appearance, while still keeping the truck features, while becoming even further realistic in the fifth season. The buildings in Axle City are also constructed with more shading and detailing so they no longer appear as just a photorealistic image in background shots.
Bob's Burgers: It's clear just from watching that season 1 was made in flash animation and was pretty rough, using a number of cut corners to get made. Season 2 has a better animation as a new tool is used.
The Boondocks' art style changes every season. The character designs that have changed the most are Sarah Dubois, Ed Wuncler, and his grandson Ed Wuncler III. Sony Pictures hired a new animation studio for the latest seasons.
The original YouTube pilot for Breadwinners didn't have very detailed backgrounds, and had a generally generic, sketchy feel. When the series received its own webpage on Nickelodeon's official website, there was also a remake of the original short. The remake featured more detailed backgrounds and more unique retro-video game style textures. The voice acting was re-recorded and it's better, to boot.
Butterbean's Cafe has much smoother and brighter animation in Season 2. It's mostly noticeable inside the cafe, and more obvious with Butterbean and Cricket.
The major cause of the broken base in Care Bears: Adventures in Care-a-Lot era of the Care Bears, where the bears were redesigned to look less chubby. The redesign itself was thrown out the window when Hasbro snatched the franchise rights away from Play Along Toys.
Casper: the friendly little ghost's first appearance showed him more chubby and duck-pin shaped; his adventures soon shifted him into the slimmer, round-headed ghost we know today.
Chaotic: The first season is animated in flash; this was an awkward style to some people. Then the second season was more like Afro Samurai with the colors of Avatar: The Last Airbender.
Chowder had completely different designs in the first episode, "The Froggy Apple Crumble Thumpkin". Mung had bigger ears and a smaller nose and eyes, Shnitzel has smaller eyes, and Chowder had a smaller face. In newer episodes, Mung has a bigger head and Chowder is thinner and has bigger eyes.
In The Critic, all the characters were given updates to their character designs. Jay Sherman and his sister Margo had the most changes.
Most of the characters (usually the main teenagers) started off looking like stick figures and in certain episodes, loose and gangly. Later, they gain thicker muscles and retain a more solid structure, sometimes to the point of stiffness.
In season 3, the fight scenes were better and improved. The first 2 seasons often showed basic punches and Ecto Rays with an irritating overuse of onomatopoeia comic book-ish freeze frame. Season 3 made it better by reducing the last and bumping the others; the results are often surprisingly fast and fluid.
In the Oh Yeah! Cartoons shorts of The Fairly OddParents, the characters' appearances were slightly different from those in the show itself. For example, many of the characters had stretched heads and bodies, Wanda's skin tone was a little more pink, and Timmy's eyes were always crossed. In an issue of Nickelodeon Magazine, Butch Hartman, the creator, mentioned that Timmy looks much cuter than when he first appeared.
About halfway through Season 10, the series switched from traditional animation to Flash animation, and outsourced all of the storyboarding and animation to Elliott Animation, a Canadian animation studio most known for animating the Total Drama series. The same studio also animated another one of Butch Hartman's shows, Bunsen Is a Beast.
Another show with quite obvious change is Family Guy:
The attention to detail in the backgrounds and objects improved over time. In one of the commentaries, the writers noticed how the pancakes in one scene are so crudely drawn that it looks more like a beehive or a stack of huge Honeycombs cereal. The food in the more recent episodes look more like real food now.
Parodied when Brian and Stewie go five years into the future and everything is 3D animation (as well as being nuclear ruins, but that's another trope). They also go back to 1999, the same year as the pilot episode and are shocked to see how different things look back then. They even poke fun at the Off-Model issues the series suffered in its early years, such as one of Peter's eyes is drawn over his nose.
Background characters are also drawn with great detail, including the dancers for the intro, where there are characters from the series appearing as the dancers instead of generic characters.
Fish Hooks shows more smoother, less choppy animation from the second season onward, one change particular being everybody's eyes are noticeably bigger than the first.
Hey Arnold!: Season 1 looked much cruder compared to the other seasons (especially see Helga's face compared to the later seasons). Season 4 and 5 looked dramatically different from earlier seasons due to the switch to computer coloring.
Home Movies stopped using Squigglevision after its first season, and switched to the much more useful Adobe Flash from there on out, resulting in much cleaner designs and actual character movement.
Honey Halfwitch, an otherwise short-lived series from Paramount Pictures Cartoon Studio, went through an extreme character redesign near the end, where she went from looking like a Harvey Comic character (she was created by Howard Post, who drew for Harvey) to looking like a girl Charlie Brown. What happened was that Paramount fired Post and replaced him with veteran animator Shamus Culhane, who ordered to have the character redesigned.
In season one of KaBlam!, Henry and June were very crudely drawn. Starting from season two, their appearances got cleaned up, and they finally got eye colors.
While Kevin Spencer is not known for its amazing animation (and occasionally lampshaded, being that they're a low-budget show), the first season was so ghastly that nothing was ever on model at any point. By the second season, the animation improved. The series got better in terms of animation, which can best be seen with Anastasia's hair becoming more realistic.
Season 2 of Kidd Video had a drastic change not only in the character design, but backgrounds as well, giving the show a more Disney-esque look than the more traditional designs of the first season.
Monique went from having black dot eyes to white eyes with pupils by the second season.
Also, the characters had more movement in the first two seasons than in later ones. The animation also got smoother as time went on.
Shows like King of the Hill have a large change in art quality from early episodes to later episodes. The pilot is crudely drawn and looks very dated. There is some improvement as season one goes on, but the change overall in how the series looks throughout the show improves later on.
Buck Strickland's design has changed the most as in his first appearance he is very short and fat and has a different shaped head, by mid season 2 he is taller a bit thinner but with a pot belly and a thinner different shaped head.
Mainframe Entertainment improved their hardware several times as technology advances over the years, and the results were noticeable between seasons of their various shows.
In ReBoot, one hardware upgrade in mid-season two allowed for much more variety of motion than they had before. Season 3 showed another upgrade with much more texture like eyelashes, partially because it wasn't produced until a year and a half after season 2 ended. Season 4 was the Un-Canceled season three years later and the characters had a much greater sense of weight.
Beast Wars had a major jump at the beginning of season two, with the introduction of the "transmetals" the robotic appearances looked better then ever. Beast Machines featured some amazing and lush explosion effects as well as a decent range of emotion from at least one character that did not have a face.
Just in Beast Wars, the CGI evolved a great deal through its three season run, as the bare and flat plains with occasional rock formations gave way to lush jungles, deep forests and underwater environments. Even non-robotic characters were way ahead of previously featured organics in terms of visual appeal: Just compare the great looking Cyber Raptors from season 3 to Dinobot's original beast-mode that had visible "seams" where his body parts met, with a terribly segmented tail, and constantly deformed in awkward ways when he moved. Or the saber-toothed cat from season two to the tiger model from the first season, which also tended to look like separate body parts stuck together. None, however, evolved as much as the butterfly — although the outdated model did appear at later points, even when the better version had already been featured.
Metalocalypse was fairly consistent in style in the first two seasons with lines getting smoother, motion becoming less stilted and clunky, and much better coloring and effects over time. Season 3 was a huge shift from the first two. Thanks to an increased budget, the characters now have pretty full range of motion compared to the first two seasons, where most of the action involved the band members standing around and talking.
Many classic cartoons have gone through design changes; Mickey Mouse is probably the most famous, but his was rather subtle evolution, being tweaked here and there as animation techniques were being refined, such as giving his eyes movable pupils. A much more obvious design change can be seen in Donald Duck who looked like this◊ when he debuted... a big difference compared to this◊
One noticeable change is that unicorn horns now are a part of the head instead of being separate, and each individual unicorn's magic is colour-coded instead of being a generic faded colour. Another is that Applejack's freckles don't vanish when she runs anymore.
In all of Princess Luna's appearances after the pilot, including the Pensieve Flashback in the Season 4 premiere, her model is radically different from that seen in her debut after being depowered from Nightmare Moon form. Fans wanked that she looked like this because she was left in a weakened state from being cured by the elements, and the writers basically said sure, why not?
The whole setting is also much more clean in later episodes. The crowd scenes in early episodes, for example, feature groups of near-identical ponies who just stand there motionlessly, aside from occasionally blinking their eyes. In later seasons, the animators have given background ponies new animations and movements so they appear more lively instead of being just filler.
The animation has gotten a lot more dynamic over each season, taking advantages of angles and momentum to give the show a more realistic look overall. Compare the first episode to the latest; basic walk cycles are hardly used anymore.
As of Season 4, the animators have begun going absolutely wild with facial expressions. You'd be hard pressed to find a single episode that doesn't haveat leasta couple.
The show zigzags this in regard to how baby ponies are drawn; the show has three separate designs for baby ponies, and tends to switch between then depending on how expressive they need to be. note The first baby shown, seen in "Mysterious Mare-do-Well" looks like a chibi-fied pony withBlack Bead Eyes. Then, later on, the Cake Twins are introduced, with colored eyes and slightly less exaggerated proportions. Then things start to get weird: "Apple Family Reunion" shows Applejack as a baby... reverting back to the initial black-eyed big-headed design. Then in "The One Where Pinkie Pie Knows", we see Applejack's birth certificate, which contains another picture of baby AJ that uses a completely different design with large, fully-detailed eyes. Then "The Crystalling" shows Princess Flurry Heart with the same large, detailed eyes. This could be chalked up to Art Evolution, but then "On Your Marks" shows the Cake Twins again, and they still have their original designs.
Body and head shapes have gotten much more diverse. Originally, the only real differences between ponies were their colors and their mane- and tailstyles, plus muzzle differences between mares and stallions, and maybe eye shape; every character had the same body shape. Now, there are fat ponies and thin ponies, tall ponies and short ponies, and facial differences are much more prominent. In some cases, there are even differences in the ears, particularly with Saffron Masala and Coriander Cumin in "Spice Up Your Life".
The early Peanuts specials have Sally wear a blue bow on the curly part of her hair. Later specials got rid of her bow, probably because it was too tedious to animate.
The Powerpuff Girls started using digital animation on and off over the course of the third season's run, before permanently switching with "Keen on Keane" late that season. The animation is noticeably not very good when zoomed in for the first few episodes that used digital coloring, but by the next season opener, "Monstra-City", the animation is a lot cleaner.
In the sneak preview for Recess, the characters (aside from Ms. Finster and Mikey) look very different than they do in the series proper.
During season 3, some episodes were done in digital coloring as opposed to season one and two's coloring which was only hand-painted cels. The digital and hand-painted would often go back and forth with mixed results until the final digitally colored episode, "Bonky Fever", in 2000.
T.J. had a slight shifting design throughout the series. In season one, he was a bit chubbier, shorter, and had highlights in his eyes. Starting in season two, the highlights were gone, but were sometimes shown in more Off-Model moments and on DVD and video covers. In season four, his eyes got bigger, and in season five, he began getting taller and skinnier.
There were also subtle character design changes as well; sometimes Ren and Stimpy would look more like their normal, original incarnations, but now they look more off-model than before. Ren's ears got shorter, Stimpy's three head hairs longer. They looked a lot more fluid, but were coloured differently, with brighter, more violent colours.
Rugrats began very crudely drawn, with rather grotesque, angular, exaggerated shapes and muted colors. By the end of the series, it featured smoother lines and more realistic coloring.
The Simpsons has lampshaded its art evolution since The Tracey Ullman Show shorts a few times. In the episode "Lisa's Sax", the early shorts are described as "crudely-drawn filler material". These shorts were so poor because Matt Groening quickly sketched his character designs to serve as inspiration for the animators, but they copied the designs exactly.
The standards of art and design in The Simpsons were mostly tied to the budget. The initial animation team consisted of four men: Matt Groening, who did (very crude) sketches and storyboards, and just three animators at Klasky-Csupo. As the series progressed, and ratings remained high, the budget, the team, and its capabilities increased. When The Tracey Ullman Show finished, Fox wanted to spin off The Simpsons as a half-hour program series, and asked Groening to make a pilot. Not wanting to go to the effort of making a pilot, which might be shelved without ever being aired, Groening refused. If the Fox executives wouldn't approve a full season based on past performance, he said, he would rather leave them and go back to his popular Life in Hell strip. In the end, they compromised, agreeing on a half-length season of 13 episodes. When this season had aired with ratings success, they got a full budget for a full second season and were able to improve the animation qualify and spend time firming up the designs.
Don Hertzfeldt's Couch Gag parodies this trend to horrifying effect — first Homer fiddles with a time-traveling remote that viscerally morphs him back to his crude 1987 model, then in a panicked attempt to undo the rewind he launches into a far future where he and his family are grotesque mutant caricatures that barely resemble their original selves.
This scene from the Season 2 episode "Bart Gets Hit By a Car" features a parallax zoom in effect. Because of technological limitations in 1990, the compositing of the scene appears visibly crude. The effect is reused in this scene from the Season 17 episode "Kiss Kiss, Bang Bangalore", and thanks to more than a decade of technological advancement, the compositions have a cleaner look to them.
Sofia the First shows smoother, much more shaded animation from Season 2 on.
South Park: The show features a lot of this over 20+ years. It started as an actual construction paper animation and moved to computer animation after the pilot. While the crude art and animation were originally part of its charm, over the course of its life, the show has gradually abandoned trying to replicate their original paper aesthetic, increasing the complexity and realism of its art and animation. Specific examples and notable lampshading include:
The show originally only showed characters from cardinal directions, often facing front and bouncing up and down to walk rather than turning and moving their legs. In later seasons, the art is given more depth, characters are shown from more angles and more often walk normally. Some dramatic sequences will actually show comparatively complex, three-dimensional animation, such as cop cars spinning as they skid to a halt.
Later-season backgrounds are a lot more detailed, with the buildings actually having perspective and lighting.
Character design has become more complex. While most kids still use the same basic model, there are now many more unique character designs. As for adult characters, you can easily tell which characters debuted in early seasons compared to those who showed up later. For example Officer Barbrady from the first season is much more simple and cartoony than the detailed and realistic Officer Yates, who debuted in season eight.
The depiction of celebrities has evolved from crude representations to more realistic caricatures. Compare the appearance of Geraldo Rivera in the first season to his appearance in the tenth season here.
Whenever the show recreates an episode from an early season, this trope becomes immediately apparent.
"Free Hat": Not in the plot itself, but features a fake commercial for a "digitally remastered" version of "Cartman Gets an Anal Probe" (the first episode), also featuring a CG spaceship and aliens, plot changes, and Imperial Walkers in the background (the whole thing is a parody of the Star Wars remakes). Notably, the animation style is a huge improvement over the original version...by being about on par with the style of "Free Hat" itself.
"A Very Crappy Christmas": One of the subplots features the kids making a cartoon about Christmas. They base the characters on themselves and do all the animation in construction paper. The joke is that the entire process is similar to how Trey and Matt first created the South Park characters to begin with, and the few scenes shown in the episode are lifted directly from their animated short "The Spirit of Christmas - Jesus vs. Frosty". Because the short is completely unaltered it ends up looking like a crude cartoon compared to the episode it appears in.
Season 7 begins with a "repeat" of the pilot episode's opening scene using the show's current-at-the-time animation and rendering techniques.
Season one looks very different from the rest of the seasons. It is the only season that used hand-painted animation cels (beginning with season 2, the series switched to digital ink-and-paint), and the animation is noticeably more crude and inconsistent as well as having a darker color palette. Also, Patrick's appearance is slightly different (mainly his eyebrows).
Episodes after the first movie had a brighter color palette. As the post-movie era progresses, the characters started getting less expressive, making their design less off-model. This image◊ comparing the 1999 and 2014 designs of the titular sponge best sums it up.
Although in season 6, for some reason, Aaron Springer decided to draw weird facial expressions especially for SpongeBob (who was notorious for those huge cheeks in certain episodes like "Boating Buddies"), but this was dropped by the following season. Also, in this season, there were more grotesque artwork like showing more veins and ripped skins.
As of "Lost in Bikini Bottom", the show goes back to having more cartoony facial expressions like it had from seasons 1-5 instead of staying on-model. Also, the outlines are thinner than ever.
The Spot the Dog animated series has went through this several of times. Between the first season of the 1987 series to the last season from 2000, Tom the Crocodile loses those menacing incisors and Helen the Hippo loses those eye sacs. And everyone changes to a lighter shade of color they were originally in. Although the changes itself could be reflecting the art evolution of the books, though.
Star Wars: The Clone Wars. The series began with very high quality 3D CGI animation and a visually appealing art style and character designs, especially for an All-CGI Cartoon on a strict TV budget. However, the art style and animation, while visually appealing, initially made the characters look slightly like mannequins and, outside of the fight scenes, move kind of stiffly. The second half of the first season and especially the second season onwards improved upon the facial expressions as well as the character movement and the second half of the third season featured a Jedi costume switch from (easily animated) body armor and gauntlets to the tunics they are seen wearing in the theatrical films.
The character designs in Steven Universevary depending on the storyboard artist, but even with the various differences in style, there are still certain things that stand out. Overall, the characters are a lot rounder in Season 2 than they were in Season 1; Pearl's face in particular is more rounded compared to Season 1.
Superjail! received a more fluid and expressive animation style with its second season, although fan reaction was mixed (to say the least), as some felt making the animation too smooth ruined the rougher look present with Augenblick's episodes. The lines also became bolder and color palettes seemed brightened, while the broader focus on story meant that the crew focused less on the wild scene transitions and used more traditional fades and cuts within some episodes. To relieve some of the criticism, the crew worked to reinstate more camera angles and transitions within season 3's episodes.
Originally the series looked quite angular and cartoon-y, with simple backgrounds and rather basic coloring. It kept the same basic style, but subtly evolved to have smoother lines, much more detail, and more natural coloring.
Also, in the early episodes, Beast Boy disappeared just offscreen to transform into whichever animal he was going to use. In later episodes, Beast Boy's transformations were on-screen. It starts some point within the first season, very awesomely in "Apprentice" which for a freezeframe you got Beast Boy as half-ram and half-snake.
Occurred constantly during the last 3rd of the second Teenage Mutant Ninja Turtles series. After 5 seasons of consistent designs, the series' art style was dramatically changed for its season 6 retool, which resulted in simpler, more angular designs, and a less subtle color palette. Then, after the series was retooled again for it season 7, most of the regular characters were redesigned to look more like their movie counterparts—most notably, the turtles' eyes now had visible irises, even when they wore their masks—and the color palette became somewhat more dark. Afterwards, for the Grand Finale movie Turtles Forever, the art style was tweaked yet again, with the turtles returning to their season 6 models (albeit lightly modified) and with several other characters—particularly Splinter and Hun—getting slightly more complex shading.
The '80s cartoon also went through several changes in art throughout the series.
Tom and Jerry were originally drawn fairly realistically, especially Tom, who had realistic cat's claws and moved and acted much like a real cat. When Hanna and Barbera figured out that slapstick involving a realistic cat doesn't work so well, Tom became more anthropomorphic.
The Venture Bros. subtly but noticeably steps up the animation at the beginning of each new season.
Woody Woodpecker went through some of this. The early entries in the series were very crudely drawn, and Woody had a ghoulish avian design. By the 10th short, when Shamus Culhane took over direction, he simplified the backgrounds, and gave the series a more or less refined art style (as refined as that low budget Walter Lantz studio could get, anyways) that would continue changing from there on out.
From the jump from webseries to television cartoon, Making Fiends got noticeably cleaned up. The backgrounds are a lot more detailed, the characters are no longer scribbly, and background characters are given brighter colors. For a comparison here's one of the protagonists house◊ in the web cartoon and TV cartoon.
The animation on Regular Show grew smoother over time. The evolution is most evident on "First Day", which is an extension of the original Cartoonstitute pilot new scenes were drawn in to extend it from seven minutes to 11, and these contrast strongly with the original. The facial expressions were more cartoonish in the older episodes as well.
Pepper Ann switched to digital coloring from cels in its second season.
Phineas and Ferb: The show sports smoother, shadier and more detailed animation from the second season onward. Notice how the characters' expressions are no longer exaggerated (especially Candace) and their movements are less choppy.
Doug had more rubbery and crude animation during its first season (and the pilot was done in Squigglevision). Season 2 smoothed it out a little more. When the show switched to Disney, the colors were more vibrant.
Many of the character designs were refined in Season 2 of The Amazing World of Gumball. This is most noticeable in the designs of the main characters' eyes, but other significant redesigns include Penny's body shape being more curved like an actual peanut (and how her mother and sister appeared in the first season) while Miss Simian's limbs and body are far less emaciated.
Transformers: Prime started off with some wonderfully detailed character models, especially the robots, but was a bit more lacking in the environments and having anything close to filling out a human-centric scene (the same three background characters are seen at the local high school every time). Lastly there was a lot of recycling of locations, almost every fight took place in a desert canyon of some sort (or arctic canyon). Season two immediately started opening things up with a trip to Cybertron and from there continued to offer a new variety of locations and better detailing to others (like some foliage surrounding desert canyons).
The Terrytoons character Gandy Goose first appeared black-and-white with a very 1930s-ish rubber-hose design (see "G-Man Jitters"). After Executive Meddling forced the studio to beef up their quality and switch to Technicolor, Gandy was given a redesign to how we know him today.
Mighty Mouse looked very generic under his original name Super Mouse, and his costume was red and blue, very similar to Superman. "The Wreck Of The Hesperus" began a slow refinement to the more recognizable Mighty Mouse, with his costume now yellow body suit with red shorts (the banned cartoon "Eliza On The Ice" had him in a red body suit and yellow shorts).
The Fleischer Studios shorts from 1929 to mid-1930 looked very crude from the usual style. There were no gray shades, the characters looked stiff and the animation was somewhat limited. It wasn't until animators like Grim Natwick and Shamus Culhane arrived that the animation vastly improved, gray shades were added, the backgrounds had more dimension and the animation moved in the studio's trademark fluid and surreal fashion.
In the second season of The Looney Tunes Show, the characters' outlines were colored and now look a little more like how they appeared in the classic shorts. For example, Bugs is now gray, and his body is taller and thinner. This is possibly in response to the massive backlash fans gave the creators who never stopped pointing out how "wrong" Bugs looked. Even Lola has gone through a little bit of change - between Seasons 1 and 2, her design became more rounded (to match the look of the other characters reverting to their traditional looks), she's now shorter than Bugs (much like her Space Jam version), she's much more curvy (her chest is notably larger in Season 2) and her dress changed from purple to cerulean. Also, the animation style in this season resembles The Life and Times of Juniper Lee.
The Hero Factory mini-movies changed their animation studio for the episode Invasion from Below from Tinseltown Toons to Ghost VFX. Although the design of the Heroes has changed throughout the specials (seeing as each one had to advertise new toys), the style stayed consistent. But for this episode, the CGI's look became less rugged and more clean, with the Heroes receiving creepily smooth faces and human-like eyes with dark pupils in place of their glowing, robotic pupils.
The first two ChalkZone shorts that aired during season one of Oh Yeah! Cartoons looked very different from the show itself. The ChalkZone itself was more simplistic, Snap had smaller eyes and wore darker shades of blue, the coloring was a bit darker, and Rudy looks completely different (though this is justified as he was two years younger). In the second season of Oh Yeah! Cartoons, the animation became more fluid, Rudy was re-designed and aged up to ten (around the same time the shorts made for the second season were being produced, Nickelodeon wanted to spin the show off into its' own series, but requested that Rudy had to be a little older for the show itself), Snap looked more like he does in the series proper, the colors were brightened a little, and ChalkZone itself is a little more detailed. The artwork and the animation became more refined and less off-model in the show itself. Then in the fourth (and final) season, Rudy and Penny received new outfits, the real world characters and background elements gained colored outlines instead of the typical black outlines used since the beginning, ChalkZone itself became a lot more detailed, and the colors became more vibrant.
The jump in quality is especially noticeable when comparing the shorts from the second season of Oh Yeah! Cartoons and season one (which most of the shorts were re-aired during) to season two. In the first season (done by Galaxy Digimation, Inc. and Rough Draft Studios), Rudy and Penny are Noodle People (especially Rudy), their movements are a lot more rubbery, and Penny's hair is shorter. Starting from season two (done by Sunwoo Entertainment and Wang Film Productions), the two are a bit more normally proportioned (at least for the art style), Penny's hair is a little longer, and their movements are smoother.
The second season of X-Men noticeably streamlined the character models to make them easier to animate. Several episodes of the fifth season were also done in a more fluid, cartoony style reminiscent of Disney animation; these episodes also tweaked a few character models, such as Jubilee getting longer hair and Apocalypse suddenly having glowingred eyes and fangs.
The Van Beuren Studios cartoons started gradually changing their design sense in 1933, and eventually underwent a complete animation and art overhaul during the 1934 period. As early as "Sinister Stuff", the characters become noticeably less flat and more rounded and appealing. Then Burt Gillett got to the studio, and the animation improved even more.
In the '90s British animated series The World of Peter Rabbit and Friends, which would have two stories in one episode or sometimes a full-length story in an episode based on the works by Beatrix Potter, the facial expressions for some of the animal characters started looking more expressive when it got to the later episodes starting on a full-length episode based on The Tale of Samuel Whiskers. Most notable when the characters get upset or angry.
The first episodes of Jem had a cheaper budget than later episodes. They're far more Off-Model than future episodes. It's also noticeable that several character designs outright changed, most noticeably Clash went from having red hair with highlights to completely purple hair randomly.
Madeline underwent a notable evolution from what was seen in the original picture books to what viewers got on the animation. The style also changed during the series itself.
Shimmer and Shine's first season was done in Flash. For season 2, the animation has been changed to CGI.
Uncle Grandpa tweaked its art style a bit in the second part of season 3, adding shading to the new intro, backgrounds and title cards, and the UG-RV is now rendered in CGI. The characters remained unchanged.
Jimmy Two-Shoes: Season 1 used ToonBoom, albeit somewhat on a low budget (hence the frequent assumption it's Adobe Flash), but in the final 6 episodes, the series changed for a lighter color palette (Lucius and Beezy become a lighter shade of red) and introduced some more detailed facial expressions. Season 2 saw a switchover to Flash, which increased the usage of detailed faces (including many very grotesque ones) and maintained the palette change. The animation also became more detailed in other aspects, such as character design and backgrounds, while character movement became less bouncy and energetic in favour of having smoothness and detail. Compare the Season 1 intro with Season 2's.
Early '60s shorts of the Hungarian adult cartoon series Gustavus had a very UPA-inspired, angular look with lots of edges, thick lines, different-colored backdrops and a bouncy animation style. Characters were simple, stylized and Gusztáv was straight-up cutesy, with a head bigger than his torso and huge eyes, which clashed with the occasional adult themes and Black Comedy. Art and animation style and quality fluctuated a lot through the years, leading to Gusztáv adopting more realistic proportions (though still keeping the noodle-thin limbs), small beady eyes, and his world became more depressive looking, with mostly the same grey-blue-turquoise backdrops, thin, uneven lines and less lively animation.
Archer is a more subtle example than most, the art style is still broadly the same but has become much sleeker and smoother over the years. They've also added more fine detail to the props and backgrounds as well as more subtle lighting that greatly improved the atmosphere. Meanwhile the character animation has become more fluid and less stilted.