According to Scott McCloud in Understanding Comics, the very medium of comics, which most people assume began with Rodolphe Töpffer in the mid-1800s. Instead, McCloud includes everything from comic books to Egyptian tomb paintings, making the medium Older Than Dirt. However, he does this by using a broad definition that is far from being universally accepted.
One of the most plagiarized concepts in comics are the misbehaving children playing tricks on adults, teachers and policemen. In Flanders De Vrolijke Bengels and De Lustige Kapoentjes popularized this concept in the late 1940s, but both comics owe a lot to Quick and Flupke (1930) by HergÚ, who in his turn was inspired by The Katzenjammer Kids (1893), who in his turn just stole the characters and set-up from Max and Moritz (1865).
Most people point at Willy Vandersteen of Suske en Wiske fame for popularizing comics in Flanders. But way before Vandersteen started creating comics during the 1940s Flanders already had local comic strip artists in the 1920s and 1930s. It's just that all of these have been forgotten nowadays. Similarly the Netherlands also had comic strips artists almost half a century before Tom Poes became popular in the 1940s.
The notion of superheroes is associated with the comic books about Superman and Batman, which popularized it in the late 1930s and early 1940s. But before them you already had the inhumanly strong Popeye, created in 1929. And many mythological stories from The Antiquity also talk about powerful heroes, just think about Heracles, aka Hercules.
The cover of Uncanny X-Men #136 (Cyclops holding Phoenix) may have been inspired by an older work of art, but it is sufficiently different from Michelangelo's Pietàs to be considered in a different category from them. (For starters, Cyclops is standing, not sitting). And the Pietà representation of Mary and Jesus had itself been used by plenty of sculptors for 150 years before Michelangelo was even born, to say nothing of not dissimilar artistic representations of other mothers with dead children dating back to pre-Christian times (e. g. Eos and her son Memnon, Niobe and one of her daughters).
Wizard magazine, the most "mainstream" magazine on comic books, once contemptuously referred to the immortal supervillain Vandal Savage as "a cheap Ra's al Ghul knockoff". Actually, Vandal Savage predates Ra's by 28 years — 1943 and 1971, specifically.
The year before Vandal Savage appeared, America's Greatest Comics had Bulletman fighting the Man of the Ages, a man who had been causing evil for a million years.
Ming the Merciless (1934) has been referred to as "a cheap Ra's al Ghul clone" as well. Ra's and Ming are knockoffs of Fu Manchu, who wasn't the first Yellow Peril villain either.
It's a meme among Fantastic Four fandom that Trelane from the Star Trek: The Original Series episode "The Squire of Gothos" was ripped off from FF villain Infant Terrible, apparently for no more reason than being child-like and omnipotent, even though there are hundreds of examples of this combination going back at least to the 1920s.
Many people seem to think that Aquaman came before Namor the Sub-Mariner, even though Namor predated him by two years. This may be because Namor went out of print during The Interregnum, while Aquaman held on as an Action Comics backup. Even then, both characters are predated by the Shark, a similar aquatic hero who has since faded into obscurity.
The DC Comics skull-faced supervillain Doctor Destiny is often called a knockoff of Skeletor, despite the fact that he predates him by several decades. Besides appearance and the fact that they're both villains, these two characters have absolutely nothing in common. And a skull face is not exactly a new or unique concept to start with.
a man "of iron" with skin so tough that it can withstand anything short of an exploding artillery shell;
able to jump so high it's like he's flying;
imbued with a high morality and sense of justice;
who hides his true ability from everyone;
fights a wrestler for money;
singlehandedly builds a fortress in a wilderness;
has adventures where he lifts a car, and rips the door off a bank vault...
The character is Hugo Danner, Gladiator, from a book published in 1930, before Superman or Spider-Man. Siegel & Schuster have admitted to taking inspiration from it when they created Superman.
Even better, Marvel later created a character named Gladiator as a Captain Ersatz of Superman for their own universe, bringing it full circle. Read more about that here: 
DC, which published a few stories of the original Hugo Danner (as did Marvel), gave Danner a son named "Iron" Munroe who filled in for Golden Age Superman in the Retcon patchwork that The DCU's World War II history became Post-Crisis, when many of the formerly Golden Age heroes were given new, recent origins. Now, the adventures that happened to Superman During the War, mostly happened to Munroe instead.
A number of comic book fans commented that the spaceship◊ in the European comic book Valerian was plagiarized from the Millennium Falcon. Except Valerian and his ship were created in 1967, and Star Wars was made in 1977.
Others commented that the flying cars seen in one of the Valerian stories were plagiarized on The Fifth Element — forgetting that the idea of flying cars has been around since... well, the invention of the car. The story was published years before the movie was made, and the director is a Valerian fan who specifically asked the series' artist to work on the design of the movie.
Ultimate Spider-Man contains an in-universe example when Peter finds a video from his father discussing how he'd planned to use the Venom symbiote to cure cancer.
Richard Parker: The first recorded mention of cancer is around 1600 B.C. Egypt. A lot of people don't know that. They think cancer came along with cigarettes and food preservatives.
Some have claimed Watchmen's ending, in which the world unites against an alien threat after New York City is destroyed, is an attempt to capitalize on post-9/11 feelings. However, not only does Watchmen predate 9/11 by 15 years, but the eerie similarities between 9/11 and Watchmen's climax have been noted by more than a few people, especially in regards to whether Ozymandias' plan to bring about world peace would work even temporarily in real life given that world sympathy for the United States was temporary and only lasted until the United States invaded Iraq.
An in-universe example has Superboy saying to Superman "Second star to the right and fly till morning." When Superman says "Peter Pan. How appropriate." Superboy replies "What are you talking about? Captain Kirk said that." in reference to Kirk's closing line at the end of Star Trek VI where he was clearly quoting Peter Pan.
In fact, Word of God has it that virtually all of the characters Moore created for Watchmen were originally to have been Charlton Comics superheroes that DC had recently purchased the rights to; when he couldn't get permission to use them (i.e. the Question), he created pastiche versions.
Many fans of misfit superhero teams who are hated and feared by the public they protect and are led by charismatic wheelchair-bound men often think that DC's Doom Patrol is a blatant ripoff of Marvel's more popular X-Men. Other comics fans who know a little more about the books' histories know Doom Patrol was actually published first, and assume the theft went the other way around. In fact X-Men followed Doom Patrol by only three months, and given the lead time involved in the production of comics it's most likely no plagiarism was involved. However, some artists and writers worked clandestinely for both companies, and it is possible that information flowed one way or the other.
Well, when one property's Rogues Gallery is The Brotherhood of Evil and the other's The Brotherhood of Evil Mutants, it's pretty obvious SOMEONE was reusing ideas.
Except that both Brotherhoods debuted at the exact same time. Both showed up in March of 1964. Yep. Same month of the same year. Making it pretty damn hard for either to be a rip-off.
Stan Lee spoke at the promotion of his "add a caption to a photo" book in 2009. In it, he specifically said that he created the X-Men because his publisher (maybe wrong title) wanted something to compete with Doom Patrol. (Lee wanted to call the book "The Mutants," but his boss over-ruled him.)
Lee himself has admitted that he has a poor memory, so it is possible that he is misremembering. It is all but impossible for the X-Men to have been inspired by Doom Patrol, as it usually took over six months to develop a comic in the 60's. As mentioned above, there were three months between the two teams' respective debuts, which would mean that the X-Men would have already been in the works when Doom Patrol hit the stands.
Adamantium is most famous as the fictional metal Wolverine's bones and claws are made of, and indeed the X-Men movies are credited with bringing the term to the general public. What fewer people know is that adamantium actually predates the creation of Wolverine by several years, as it was first introduced in an issue of The Avengers back in the 60's as the material Ultron's body was made of.
On the subject of the X-Men, the concept of superpowered mutants predates them even at Marvel. The second issue of Yellow Claw from 1956 features six mutants with psychic abilities who are manipulated into using their powers to help the Claw's evil schemes.
Jimmy Woo: Yes, I said mutants, chief! People with deviations...in either mind or body...or both!
On a more general focus there had been superpowered mutants even earlier in science-fiction novels, notably Stanley G. Weinbaum's Adaptive Ultimate (Astounding Stories, November 1935, adapted into the movie She Devil in 1957). And Wilmar H. Shiras wrote a series of pulp stories about superpowered mutants persecuted by a world that fears and hates them starting in 1948 under the general title Children of the Atom.
The mix of ancient characters with original creations in stories about characters like Marvel's Thor and Hercules and DC's Wonder Woman leads to an understandable amount of both Older Than They Think and Newer Than They Think. Notable examples include reviewers thinking a character was being antisemitic for calling Hercules's wife "Hebe" (that's her name, from classical Greek mythology, and it's pronounced differently to the modern anti-Semitic slur).
Brian Azzarello's wildly popular Batman story Joker, about (you guessed it) The Joker, takes place in a more realistic universe, where the Joker is depicted as a more believable psychopath. His long messy hair, splotchy "makeup", and Glasgow Grin made him a very unique version of the character.... until The Dark Knight came out. Many people consequently thought that the comic "ripped off" Heath Ledger's Joker, or that the story was outright set in the Dark Knight Saga continuity. However, this was just a coincidence, they had already started the story before they even saw what Ledger looked like.
The comparison is only appearance based however, as Ledger's Joker and Azzarello's Joker have very different personalities. And the plot of the Graphic Novel was inspired by the 1989 Christopher Walken movie King of New York.
In general, superhero comics' Darker and Edgier turn has often been exclusively credited to (or blamed on) The Dark Age of Comic Books and the trend's Bronze Age origins overlooked. (And those origins aren't just in the earlier work of later Dark Age stars like Alan Moore. Witness the hero apparently confessing to off-screen torture, then gloating over his fallen enemy as he slowly and deliberately batters him to death on screen ... in "Spawn" from New Gods ... by Jack Kirby, in 1971!)
The Joker's signature weapon, Joker Venom, debuted in the very first Batman story, published June 1940. It's often believed to be the modern western origin of the idea of a poison that causes its victims to Die Laughing. However, 3 months before Batman's debut, The Shadow aired an episode called "The Laughing Corpse" that featured a killer who poisoned his victims with a toxin that caused their muscles to contract their face into a grotesque smile and convulsions that made the subject appear to be laughing themselves to death. Of course, this isn't the first time that The Shadow informed some aspect of Batman's creation.
Neil Armstrong went to the moon in 1969. Snoopy was there earlier that year. Tintin predates Snoopy by 16 years, going to the moon in 1953. Donald Duck already went there in 1948. But Jules Verne's Around The Moon predates everyone (nope—see below), being published in 1870. That's 99 years before Real Life. In From The Earth To The Moon nobody set foot on the moon.
James A. Owen received a lot of hate mail for "stealing" the characters of Titania and Oberon from Neil Gaiman's Sandman series for his own Starchild series. It got so bad that, as a favor, Neil wrote the intro for the collected anthology explaining that James did not, in fact steal anything that wasn't already stolen.
Some fans have accused Spider-Man Arch-EnemyNorman Osborn (aka the Green Goblin) of being a rip off of Lex Luthor, because both are Corrupt Corporate Executives. Thing is, Norman's been an evil businessman since he debuted in 1966 (his Goblin alter ego debuted two years before he did, interestingly enough), while Luthor was a traditional Mad Scientist from his appearance in 1940 until his reinvention as a businessman in 1986.
Masked crimefighter who is actually a blind man whose work includes court trials. You are probably thinking about Daredevil, who is actually Matt Murdock, a blind lawyer. But years before him, in 1939, there was another character like that: blind DA Tony Quinn AKA The Black Bat.
Alpha Flight was launched by great fanfare with a major selling feature being it was the first major superhero title featuring Canadian characters. In fact, Captain Canuck had been running for a number of years by that point, and the Alpha Flight character Guardian bore more than a passing cosmetic resemblance to the Captain (Guardian was introduced in 1978, but Captain Canuck had debuted in 1975).
Batman ripped off Dick Tracy characters a lot, if not necessarily intentionally. (Dick Tracy predates Batman by almost a decade.) A guy with his mouth frozen open in a huge grin? That wasn't the Joker originally, that was Laffy, who had many of the Joker's personality traits to boot. (Laffy met a tragic end when he starved to death after his jaw was inadvertently wired shut in an effort to fix his face.) And whom do you picture when you're asked to imagine a guy with a comically long nose? The Penguin, right? Well, he was preceded by another Tracy villain, Ribs Mocca, who looked just like the Penguin, except for being much skinnier.
"Broadway" Bates, a Tracy villain introduced in 1932 not only has a long nose, but formal dress, a monocle and a cigarette holder. The current Dick Tracy writers have lampshaded this by claiming "Broadway" has a brother called Oswald in an unnamed city known for costumed heroes...
A villain whose modus operandi is leaving riddles. Sounds like the Riddler? Actually before that was Doctor Riddle, a hunchbacked Bulletman villain who first appeared in 1942, while the Riddler first appeared in 1948.
Cassandra Cain was the first Asian-American member of the Batfamily (and the Batfamily's first hero of color in general), as well as DC's first Asian character to have her own ongoing series. About a year or so before she debuted, John Byrne pitched an idea for a new Asian-American Batgirl, though it never came to fruition.
The superhero's arch-enemy, a bald scientist, turns out to have to have a tragic backstory, where he was originally trying to use his inventions to do good. That is not referring to Lex Luthor, it is referring to Doctor Sivana, arch-enemy of Captain Marvel. This was revealed in Whiz Comics #15, from 1941.
Speaking of Luthor...In 1939 Superman gained a bald arch-villain. His name? The Ultra-Humanite. Lex Luthor was created in 1940, and his signature Bald of Evil look didn't appear until later. (In fact, the change in Luthor's design is believed to have been an accident.) The Ultra-Humanite himself may be based off a short story made by Superman's creators in 1933, titled "The Reign of the Superman".
Ever heard of that superhero who has mechanical bracelets full of web fluid and uses them to shoot web lines to swing around on and catch bad guys? Of course I'm referring to Fox Features' The Spider Queen, first appearing in September, 1941.
Many people think, incorrectly, that Wonder Woman was the first female superhero. Actually, there were dozens of female superheroes prior to Wonder Woman. One of the earliest is Ritty, who debuted in late 1939, two years before Wonder Woman. In fact, Wonder Woman isn't even the first female patriotic superhero. She was preceded by USA, Miss Victory, Miss America, Miss Patriot, Pat Patriot and War Nurse.
The earliest (comics) superheroine quite probably is Olga Mesmer, the Girl with the X-Ray Eyes, who appeared in a back-up comic series in the pulp magazine Spicy Mystery Stories starting in August 1937. That's right, she got into print before Superman. Olga Mesmer also was super-strong and the daughter of an alien queen (belonging to a subterranean race that originated from Venus), but her feature only lasted a little over a year. Considering the title of the magazine in which she appeared, it's probably no surprise that Olga did not fight in a colourful costume, but mostly in her underwear. Fun fact: Both Spicy Mystery Stories and Action Comics were published by Harry Donenfeld.
Deadshot is commonly thought to have debuted in the '70s or '80s due to his reinventions in 1977, and having become a prominent member of the Suicide Squad during that time. He actually debuted in 1950 as a one-off Batman villain, and was damn near unrecognizable to the Deadshot we know today.
Black Panther is often said to be the first black superhero, but that honor actually belongs to Lion Man, an obscure Golden Age hero who appeared in the sole issue of All-Negro Comics back in 1947. And while he didn't have powers, the gunslinger Lobo was the first black character to have his own comic book series, while Waku, Prince of the Bantu had his own feature in Atlas Comics' Jungle Tales anthology series.
Attilan, the hidden city of The Inhumans, was first mentioned in a Tuk the Caveboy story back in 1941.
Many people think the Affirmative Action Legacy trope is a modern concept that was designed to appeal to more progressive, 21st century political sensibilities. At the earliest, they tend to think it started in The '90s with characters like Steel and Connor Hawke. In reality, the first major instance of this trope at Marvel or DC was John Stewart, the black Green Lantern, who debuted all the way back in 1971.
Related, but in the 21st century, Captain America had two very high profile instances where he was replaced. First was by Bucky Barnes after Civil War, and then again by Sam Wilson in 2014. The first major instance of this happening was back in the 80's during Mark Gruenwald's run, and even before that, J.M. DeMatteis had pitched a story where Cap would have been killed off and replaced by Jesse Black Crow, a young Native American man who had previously appeared in his run.
You might think that having Cyclops (mutant terrorist) on the cover of Rolling Stone in All-New X-Men was a reference to the controversy surrounding Rolling Stone's "glam Boston Bomber" cover; however the first comic where it appeared came out at least six months before the actual bombing, much less the magazine cover.
When Pacific Rim was released in the UK, Judge Dredd Megazine reprinted Detonator X. Cue indignant fan letter complaining about how Guillermo del Toro didn't acknowledge his inspiration from a little-known British comic, to which the editor replied that giant robots fighting giant monsters had been a mainstay of Japanese monster movies and anime for decades.
The Avengers, or an Avengers-like team, is assembled by S.H.I.E.L.D. to help stop villains instead of forming on its own. Sound familiar? Granted, it's regard by most fans as a Dork Age and hence most have chosen to forget about it, but Heroes Reborn, which saw print in 1996, was the first to use the concept of S.H.I.E.L.D. being the ones to bring together the Avengers, five years before The Ultimates did it and 16 years before The Avengers movie used it (and that was likely borrowed from The Ultimates).
Similarly, Galactus's arrival being heralded with several heralds instead of one, and each one establishing and protecting a machine at distant places of the world instead of a single machine in New York that The Avengers: Earth's Mightiest Heroes! used? Also used in Heroes Reborn, which came 14 years before.
The first post-Liefeld Captain America issue mildly retconned Cap's backstory to reveal a World War IISuper Soldier was defrosted during both The Korean War and The Vietnam War, only to put him back under when their mental conditioning started to buckle. Sounds like Bucky Barnes following becoming the Winter Soldier, right? Nope, replace the Soviet Union with the United States and you've got Heroes Reborn Captain America's backstory.
An aircraft pilot named Hal Jordan, appearing in a superhero comic book? It happened in a Golden Age Timely/Marvel comic book starring Sub-Mariner, in 1945. Coincidentally (or not), his look is very simiar to his more famous namesake's, who showed up 10 years after.
Underground Comics: Often associated with Robert Crumb, though he wasn't the first artist to drawn comics tackling sexual and political taboos. Jaxxon's "God Nose" (1964) is seen as the oldest example. And in the 1930s and 1940s anonymous comic strip artists made pornographic parodies of well known comic book characters, nicknamed "Tijuana Bibles".
Many Black Canary fans think of her as a relatively new character, maybe from The '70s or The '80s. The original Black Canary - Black Canary is actually a Legacy Character - first appeared in 1947. Black Canary was one of the first female members of the Justice Society Of America, and for that matter Black Canary had a love interest, and later husband, in Larry Lance for over twenty years before she began associating with Green Arrow.
Barbara Gordon is almost always considered the original Batgirl; however, five years prior to Babs appearing in Batman there was already a "Bat-Girl", who was a sidekick to Batwoman and more of a direct Distaff Counterpart to Robin. Betty Kane was scrapped after a while but reappeared as an adult in the late 70s, only to be retconned out of existence after the universe reboots. She was later reintroduced as "Bette Kane" and made into Flamebird. Bette is no longer considered a Batfamily member and DC ignores her run as Bat-Girl, but she still technically was the first Batgirl.
Tales of the Jedi: The comic itself is the first EU appearance of the Old Republic Era (taking place 4,000 years before any of the films), but most fans know it from the Knights of the Old Republic games, which were released around the time of the prequel films. It also contains the first appearance of the double-bladed lightsaber, the Jedi Council, and the Sith Rule of Two later seen in Star Wars: The Phantom Menace.
Dark Empire II by the same creators had a scene which showed a conference room in an ancient Jedi citadel. The room looks identical to the Jedi Council Chamber that would appear in the prequels (with the sole exception of the center of the room being occupied by a crystal monolith). This comic was published in 1995, four years before Star Wars: The Phantom Menace.
A lovable, idealistic young South Asian Muslim female Marvel superhero whose family relationships are deliberately warm and supportive, in opposition to anti-Muslim stereotype, and who is an unashamed fangirl of the older-established Marvel heroes? Could be Kamala Khan, but before her it was Faiza Hussain.
Beast Boy debuted in the 1960s but most people know him from The New Teen Titans comics of the 1980s or its 2000s cartoon adaptation Teen Titans. He was even in an original Teen Titans issue but was denied the ability to be a Titan because he couldn't get adult permission.
When The Killing Joke was adapted into Batman: The Killing Joke, the movie got flak for inserting a relationship between Batman and Barbara Gordon, many people thinking Barbara Gordon being a librarian was an Actress Allusion to Tara Strong's role as Twilight Sparkle, and that the movie having Barbara Gordon retire being Batgirl before she's shot undercuts the tragedy of what happened. Except that a relationship between Bruce and Barbara was part of Batman Beyond's backstory (though that got flak, too), Barbara was already a librarian when she was introduced, and a one-shot published shortly before TKJ, Batgirl Special No. 1, ended with Barbara deciding to hang up the cowl, so she was retired in the comic, too.
The Batman villain, Scarecrow, didn't create the Fear Gas, it was originally created by HugoStrange◊.
The decline in superhero comic books sales dates to the 90s, or possibly the late 80s at the earliest, right? Wrong, the sales figures of superhero comics have been in a steady decline since the early 1960s, with the Silver Age titles selling significantly fewer issues than the Golden Age titles. It was this decline, much more than the 1971 revision of the Comics Code, which lead to sudden reappearance of mainstream horror comics.
One of Spider-Man's foes decides not only to "kill" him, but become a Superior Successor show that they could be a better Spider-Man than Peter Parker ever could. Sounds like Superior Spider-Man, right? Well, it actually happened before, as part of Kraven's Last Hunt had Kraven pretending to be Spider-Man after drugging and burying him alive to show himself as Spidey's superior.
Synergy between comics and their adaptations. The practice really came to prominence (and controversy) in The New '10s, when Marvel began aggressively trying to sync their comics up with the movies and shows of the MCU, but it was already around way before that. Not only was it already a thing with previous Marvel movies note Among MANY other examples, Blade's enhanced strength comes from the Wesley Snipes movies, while both Mystique and Toad got redesigned to resemble their counterparts from the first X-Men film, with Toad's prehensile tongue also coming from the flick, but it has arguably been going on as long as adaptations of comic books have. Superman's flight came from the Fleischer cartoons, while both Jimmy Olsen and Kryptonite originated in the radio show.
Many people who come across The Shield comics by Dark Circle (a subset of Archie Comics aimed at darker superhero stuff than their typical fare) think that the titular superhero is a Gender Flippedexpy of Captain America. The Shield actually predates Captain America by several months, though the modern 2015 version is a new character as The Shield is a Legacy Character.