At this point, with the overwhelming amount of comic book movies, television shows, cartoons and web series' being made, it's getting harder and harder to find a significant character who hasn't been brought to the screen yet. As well as being on first name terms with the big hitters like Batman, Superman, Captain America and Iron Man, the world at large is now familiar with fairly obscure Marvel and DC characters like Groot, Steppenwolf, Cloak & Dagger, Heatwave and Captain Cold, to name but a few. It's a pretty amazing time to be a fan of comics, and the ubiquity of superhero product on-screen seems set to continue for many more years to come. After all, there are still three big releases to come in 2018 (Venom, Spider-Man: Into The Spiderverse and Aquaman) and a further nine comic book movies set to hit theatres in 2019.

It's therefore fun for fans to wonder about which of their favorite characters are going to be brought to the screen next, and in what form. But there is an interesting subset of characters that has grown over the years, as more and more of these projects are made. Plenty of movies, TV shows and animated series haven't relied entirely on the source material when it comes to their characters. In fact, some have created wholly original characters for themselves and a select few of them have proved so popular that they were subsequently integrated into the comic book continuity. It might surprise you to learn that these 20 characters originally made their debuts outside of the comics pages you know them from.

20 HARLEY QUINN

We'll start our list off with arguably the most popular comic book character to first debut on-screen. We are, of course, talking about Dr Harleen Quinzel, everyone's favourite Arkham Asylum psychiatrist-turned-kooky supervillain! Harley Quinn first appeared in Batman: The Animated Series in September 1992, in the brilliant episode "Joker's Favor".

Initially created by Paul Dini in a somewhat throwaway manner, the jester-suit wearing sidekick for the Clown Prince Of Crime proved massively popular. She was eventually brought into DC continuity in Batman: Harley Quinn #1 in October 1999 as part of the "No Man's Land" storyline and went on to have her own long-running title. She was then brought to the big screen by Margot Robbie in 2016's Suicide Squad.

19 LIVEWIRE

Livewire is another female supervillain who first appeared in a Bruce Timm DC animated show. In the appropriately-titled "Livewire", a 1997 episode of Superman: The Animated Series, fans were introduced to Leslie Willis. A fading Metropolis radio shock jock, she was struck by lightning while hosting a rock concert during a thunderstorm.

The lightning was conducted through Superman's body, as he was there attempting to save Willis, and her skin was turned white and her hair turned blue. Oh, and she could now control and manipulate electricity! Livewire was brought into DC continuity in Action Comics #835 in 2006. The story was written by Gail Simone and art was by John Byrne. She later appeared in both the Smallville and Supergirl live-action TV shows.

18 FIRESTAR

Firestar flying through the city

Angelica "Angel" Jones, aka Firestar, first appeared in the Spider-Man and his Amazing Friends cartoon in 1981. The show bizarrely teamed Spidey with Iceman, of the X-Men, and the original creation that was Firestar. Producers reportedly wanted to use Johnny Storm, aka Human Torch, but couldn't sort out a rights issue. So they created a character with similar fire-related powers, but made her female.

This meant a love triangle storyline between the heroes was inevitable. She came into the Marvel comics universe four years later, first appearing in Uncanny X-Men #193. Over the years she has been a member of the Hellions, Avengers, X-Men and the New Warriors.

17 BEBOP & ROCKSTEADY

Bebop and Rocksteady are the most beloved humanoid warthog and rhinoceros henchmen in history, that much is obvious. But did you know they made their first appearance in an episode of the 1987 animated Teenage Mutant Ninja Turtles series, rather than in the comic that inspired the show? They were initially designed by Turtles co-creator Peter Laird as part of a deal with Playmates toys, who were demanding more characters to be made into action figures.

They were then given names and backstories when they appeared in the episode "Turtle Treks". They're currently experiencing a renaissance of sorts, starring in their own comic (Bebop and Rocksteady Destroy Everything) and appearing in the live-action film Teenage Mutant Ninja Turtles: Out Of The Shadows, both in 2016.

16 TERRY MCGINNIS

In 1999 the team behind Batman: The Animated Series created Batman Beyond, a new cartoon following the adventures of Terry McGinnis, the Batman of the future. Terry was a 16-year old kid who discovered an aging Bruce Wayne was the legendary Batman, and eventually donned a high-tech new batsuit and became Gotham's new protector.

All this with Bruce constantly in his ear giving him orders, of course. Terry was officially made part of DC continuity in 2014's The New 52: Future's End maxiseries, though he had previously appeared in several other future-set stories in the likes of Superman/Batman and Batman #700.

15 EGGHEAD

Batman 1966 Egghead

The '60s Batman television series was a camp delight that poked affectionate fun at its source material. While yes, it doesn't jive with most fans' vision of Batman as a grim vigilante, it was still great fun. The show contributed to the overall Batman mythology by creating several original villains. Egghead, played by horror legend Vincent Price, was one of these creations.

He had a pale bald head and wore a yellow and white suit. Get it? Like an egg! He also used a lot of egg-related puns in his speech, such as "egg-zactly" and "egg-cellent". Egghead went on to make scattered appearances in the comics, such as Gotham Academy, and his name has been mentioned in Tom King's current Batman run.

14 S.H.I.E.L.D. AGENTS

The S.H.I.E.L.D. comic book series starring Phil Coulson brings Clark Gregg's character to the pages, but Marvel also used that book to introduce a host of other characters from the Agents Of S.H.I.E.L.D. TV show to the Marvel Universe.

Agents Melinda May (Ming-Na Wen), Leo Fitz (Iain De Caestecker) and Jemma Simmons (Elizabeth Henstridge) were all original creations of the show, and the series was able to give them each a one issue introduction to the regular continuity. Other original characters from the show were also merged into the comics, such as Grayson Blair, Skeesh, Ethan Slaughter, Horguun, Detective Nicole Orr, D.E.A.T.H and the All-New Howling Commandos.

13 ERIK SELVIG

Erik Selvig tries to explain something to his friends

Astrophysicist Erik Selvig was played by legendary Swedish actor Stellan Skarsgard (Mamma Mia!, The Girl With The Dragon Tattoo) in four MCU movies: Thor, The Avengers, Thor: The Dark World and Avengers: Age Of Ultron. Loki used the mind stone to control him during his attack on New York, and even though Selvig was freed from the control, he was shown to have been deeply affected by the experience.

He made his mainstream Marvel Comics debut in the 2016 "Avengers: Standoff" storyline, where he was portrayed as a villain. A Danish doctor and undercover Hydra agent within S.H.I.E.L.D., he sacrificed himself trying to protect Kobik (the physical manifestation of the Cosmic Cube) from Steve Rogers.

12 RENEE MONTOYA

GCPD Detective Renee Montoya has developed into an integral part of the DC Universe since her debut in Batman #475 in May 1992. In the seminal Gotham Central series she was outed as gay and later took on the mantle of The Question. She also had a long-running romantic relationship with Kate Kane/Batwoman.

Her first appearance in Batman: The Animated Series was in its first episode, which aired in September 1992, after she'd appeared in the comics. However, Paul Dini actually created her for the show first, and when the comics creators heard about her, they asked if they could put her in the comics. Since it takes less time to put together a comic, it  hit shelves several months before the show debuted!

11 JOHN DIGGLE

John Diggle listening to Oliver Queen in Arrow

John Diggle has been one of the constants of DC's Arrowverse of shows since way back in 2012 when he appeared in the pilot episode of Arrow. Oliver Queen's bodyguard and a military veteran, he later joined Oliver's crusade to rid Starling City of crime. Actor David Ramsey imbued Diggle with a sense of honor from day one, and he would take on the codename Spartan from season four onwards.

Writer Jeff Lemire and artist Andrea Sorrentino incorporated Diggle into their Green Arrow comic in 2013, although their version of the character was quite different from the TV incarnation. Also, fun fact: John Diggle was named after Andy Diggle, the writer of the seminal Green Arrow: Year One series.

10 AGENT PHIL COULSON

Phil Coulson Agents of SHIELD

Phil Coulson, the unflappable S.H.I.E.L.D. agent who formed the connective tissue between the early movies in Phase One of the Marvel Cinematic Universe, was a delight from the second he appeared on-screen. Played by veteran actor Clark Gregg, he gave Coulson a demeanor of knowing humor and had a laconic delivery that fans loved.

He was slain by Loki in the first Avengers movie, but was later resurrected to star in the ABC show Agents Of S.H.I.E.L.D. He first appeared in the mainstream Marvel comics universe in Battle Scars #6 in 2012, and would go on to headline his own S.H.I.E.L.D. series, written by Mark Waid from December 2014 onwards.

9 MORPH

Morph is another strange case. A character named Changeling was a member of the X-Men, debuting as far back as X-Men #35 in 1967. He would go on to be the first one of Xavier's team to fall in action. Over 25 years later, a character named Morph appeared in the first two episodes of X-Men: The Animated Series, and he was ended by the Sentinels.

Both characters were shapeshifters, and the only reason the name Changeling couldn't be used was because DC had trademarked it. Inspired by the animated series, a new character named Morph was then brought into the comic books in the "Age Of Apocalypse" event, and then another reimagined Morph debuted in Exiles #1 in 2001.

8 THE PHANTASM

Batman: Mask Of The Phantasm is often called "the best Batman movie ever made" by fans, and with good reason. It's a tour de force of animation that works equally well for adults and children, it's got all the requisite superheroics for kids to get excited about but also tells a genuinely emotional and insightful tale that probes deep into Batman's psyche.

The Phantasm/Andrea Beaumont was an original creation for the movie (although she was heavily inspired by The Reaper, a villain from Mike W. Barr's Batman: Year Two story). Batman and Andrea's doomed romance is beautifully realized in the film, and she later appeared in the Batman Beyond 2.0 comic series, as well as DC Comics Bombshells.

7 CHLOE SULLIVAN

Chloe Sullivan, was one of the most popular characters of the hit Superman origin CW series, Smallville for the better part of a decade. Chloe was Clark Kent's best friend, an intelligent and independent woman who followed her passion and became a journalist at the Daily Planet.

She secretly pined for Clark for a few years before accepting that they would never be more than just friends. Chloe was brought into DC continuity in 2010's Action Comics #893 in "Jimmy Olsen's Big Week", where she was revealed to have shared a romantic history with Jimmy, whom she had actually married at one point in the show's continuity.

6 JESSICA PRIEST

Melinda Clarke (The O.C.) played Jessica Priest, a mercenary on Jason Wynn's payroll, in 1997's much-maligned big screen Spawn movie. It is Priest who shoots Al Simmons (Michael Jai White) after their boss Wynn (Martin Sheen) orders it while they are on a covert mission. Simmons is then set on fire by Wynn and goes to Hell, where he is transformed into a Hellspawn.

In the comics, Simmons was downed by Chapel, a character from Rob Liefeld's Youngblood series. Rights issues meant that Spawn creator Todd McFarlane came up with Priest as a substitute, and later retconned his comic book continuity to make Priest the culprit. Her backstory was then expanded upon in 1998's Curse Of The Spawn #12-14.

5 LOCK-UP

Paul Dini created Lyle Bolton, aka Lock-Up, an intimidating and unhinged man specializing in incarceration and high-tech security systems, for Batman: The Animated Series. He was recommended for the job of Chief Of Security at Arkham Asylum by Bruce Wayne, who soon saw the grave mistake he had made when Bolton turned the facility into a violent police state.

Lock-Up took less than two years to make his way over to the comics. Writer Chuck Dixon used him in Robin and Detective Comics, where he captured several Gotham criminals and tried to drown them in an underwater trap. He then appeared again during "No Man's Land", where he was shown to have taken control of Blackgate Penitentiary in the wake of Gotham's devastating earthquake.

4 NUCLEAR MAN

Superman IV: The Quest For Peace is a truly terrible movie. It was so bad that it took a full 19 years for Superman to find his way back to the big screen in 2006's Superman Returns (and that movie completely ignored the continuity established in Superman III and IV). The villain in this misfire was Nuclear Man, an evil solar-powered version of Superman created by Gene Hackman's Lex Luthor.

He was played by Mark Pillow, who had never acted before, and only acted in two more projects after this movie bombed. Nuclear Man was brought kicking and screaming into DC continuity by Brian Michael Bendis in last month's Superman #2, where he was a "terrible secret" offed in one scene by new villain Rogol Zaar.

3 MERCY GRAVES

Mercy Graves, Lex Luthor's personal assistant and bodyguard, debuted in Superman: The Animated Series in 1996. She was once the leader of a gang of thieves and she stole a briefcase right from under Luthor's nose. He tracked her down and offered her a job, as he was impressed by her ruthlessness and brazen attitude.

A woman of few words, she nonetheless displayed a biting sarcastic wit when she did speak. She joined the DC Universe proper during the "No Man's Land" storyline in 1999, where she was later joined by another female bodyguard (Hope) in Luthor's employ -- it was hinted that the two were Amazons. She then appeared in Batman V Superman: Dawn Of Justice in a small role, played by Tao Okamoto.

2 KING TUT

King Tut Batman 66

Another villain from the '66 Batman show, the other most famous original villain created was King Tut. Played by Victor Buono, he was an Egypologist at Yale University who suffered a blow to the head during a student riot. He woke up believing himself to be the reincarnation of ancient Egyptian Pharoah King Tut and then, naturally, tried to take over Gotham City.

In the ensuing tangle with the Dynamic Duo he was struck on the head again, and reverted back to his old self. Which, we are positive, is how brain injuries work. He was finally brought into DC continuity with Batman Confidential #26 in 2009 and was a crazy person who left Egyptian-themed riddles.

1 X-23

X-23 Target X

After Harley Quinn, x-23 might be the most important character in comics history to have debuted on-screen before being brought over to the comic books. Created by Craig Kyle for the X-Men: Evolution cartoon series in 2003, X-23 was immediately popular. A clone of Wolverine (and later his adopted daughter), she was born and bred to be the perfect slaying machine.

She was also a young girl struggling with the weight of her violent past. She debuted in the comics in 2004's NYX, and then starred in several miniseries' written by her creator Kyle and Christopher Yost. She later took on her father's costume and identity in All-New Wolverine, and made her big screen debut in 2017's Logan.