The image of the evil clown appears to be a recent development in popular culture in which the playful trope of the clown is rendered as disturbing through the use of horror elements and dark humor.
Many people find clowns disturbing rather than amusing. This may have origins in childhood trauma. Many children are afraid of disguised, exaggerated, or costumed figures — even Santa Claus — and an unfortunate childhood experience with a clown could have long term consequences. Clown costumes tend to exaggerate the facial features and some body parts, such as hands and feet. This can be read as monstrous or deformed as easily as it can be read as comical. At the same time, the clown act is often represented as drunken, reckless, or simply insane — that of the giggling maniac. This includes the notorious Canio who murdered Nedda and Silvio (recorded in Leoncavallo's opera Pagliacci.)
An extreme fear of clowns is known as coulrophobia. It is sometimes also referred to as caulrophobia, or clownophobia.
It can also be said one's response to a clown might depend on where it's seen. At a circus or a party, a clown is normal and one may find a clown funny. The same clown knocking on one's front door late one evening is more likely to generate fear or distress than laughter or amusement. This effect is summed up in a quote often attributed to actor Lon Chaney, Sr.: "There is nothing funny about a clown in the moonlight."
John Wayne Gacey
John Wayne Gacey, (March 17, 1942 – May 10, 1994) was an American serial killer. He was convicted and later executed for the rape and murder of thirty-three boys and men, 28 of whom he buried in his crawl space, between 1972 and his arrest in 1978. He became notorious as the "Killer Clown" because of the many block parties he attended, entertaining children in a clown suit and makeup, under the name of Pogo the Clown.
The wide publicity of Gacy's crimes is often presumed to have a strong influence on the idea of an evil clown.
The evil clown in popular culture
The image of the evil clown appears to have gained notoriety in the late twentieth century, to the extent of becoming a cliche.
Major examples of "evil clown" imagery are:
- In the Beatles' animated film Yellow Submarine, large clowns serve as artillery for the Blue Meanies.
- Pennywise in Stephen King's 1986 novel It, and the made-for-TV movie based on the book.
- Musical groups such as the Insane Clown Posse, Twiztid, and Shawn "Clown" Crahan of Slipknot who impersonate clowns in a "creepy" manner.
- Papa Lazarou of BBC sitcom, The League of Gentlemen, the Wife-stealing Circus Master of the Pandemonium Carnival. Papa Lazarou's specific make-up, however, has greater resemblance to Black and White Minstrels.
- Yucko the Clown, a frequent guest on the Howard Stern show, is a racist clown who claims to be a registered sex offender. In addition to his racist jokes, he prides himself on never washing his clown costume and his rank odor.
- Koko the Killer Clown a featured attraction at a sideshow on Coney Island. 
- In Living Color featured Homey D. Clown, a short-tempered ex-con drug-addict with a violent brand of comedy.
- The science fiction movie Killer Klowns from Outer Space.
The Joker, the archetypal clown supervillain
- The Joker who is the greatest enemy of Batman, is a murderously insane supervillain with a disturbing clown-like appearance. Appeared in Batman #1 (1940). Three decades before John Wayne Gacey.
- The clown doll in the first of the Poltergeist movies, which becomes possessed by a ghost and attempts to strangle a young boy.
- In a Treehouse of Horror parody of the Trilogy of Terror doll, Homer Simpson once purchased a talking Krusty the Klown doll which attempted to kill him. It was revealed that the reason it attempted to kill Homer was because a switch in the toy's back was set to "evil."
- Kinko The Clown song about a kid "loving" clown who sounds a lot like Gacy. 
- Zombie clowns who drowned in a circus train accident return to eat the living in Dead Clowns, a low-budget horror movie made in the US and released in the UK in 2003.
- The Chief Clown (Ian Reddington) in the 1988 Doctor Who episode "The Greatest Show in the Galaxy" is a high-voiced capering assassin with an army of similar-looking robots.
- The Star Trek: Voyager episode "The Thaw" features a mocking clown (Michael McKean) who presides over an Edgar Allen Poe influenced virtual reality kingdom. He feeds on the fear of those wired into the VR, and is determined to keep them under his power.
- Doink the Clown, the stage name of a professional wrestler of the 1990s. He wrestled in clown costume and makeup, but for the most part portrayed a heel, one who wrestles in an unsportsmanlike manner.
- Buggy the Clown, a villainous clown pirate captain with supernatural powers, from the One Piece manga and anime.
- The serial killer clown named Sweet Tooth from Sony's video game series, Twisted Metal.
- The clown doll in the video game Alone in the Dark 2, which gets animated and kills by strangling. It is very probably based on Poltergeist.
- Khan the assassin in the adventure game Broken Sword; he is dressed as a clown playing accordion who bombs a French café in the beginning.
- In Middletown Township, New Jersey, there is a giant metal sign called the "Evil Clown of Middletown", which has scared and delighted many children over the years and has been threatened to be knocked down by a town center.
- In the Spawn comics, the demon, Violator uses the human form of a fat clown called Clown.
- Superman has faced a villainous fat clown named Tringle in some comic issues.
- The character Piemon (Piedmon in the English version) from the first season of the Digimon anime.
- In the PS2 RPG Dark Cloud 2, an evil clown known as Flotsam pursues the hero with the aid of a giant clown robot called Halloween and an army of knife-wielding clowns.
- Mega Man has fought a robot master called Clown Man and an unnamed fat robot clown whose head was badly attached.
- In the arcade game Tumblepop, the first boss (in the Russia levels) is a giant clown held by balloons who juggles with bombs.
- In the arcade game CarnEvil is an evil clown head as an enemy.
- Coco Demento, one of the students in ¡Mucha Lucha!, has the appearance of an evil clown, and was even shown as a bad guy in his first appearance. Afterwards, though, he is not considered a villain by the rest of the cast, in spite of his practical jokes. It was later revealed that he once hung out with some really evil clowns whom he is now afraid of.
- Tricky, the clown from the Madness Flash series, has the basic clownlike looks (funny hair, white face, and blue paint around his mouth in a happy expression), but is a very maniacal killer when reality is compromised.
- Mr. Giggles, Mischief and Stumpy were evil Clowns in the TimeSplitters series.
- In Rockstar's Western-themed shooter Red Dead Revolver the protagonist faces off against a troupe of shotgun-wielding midget clowns early in the game.
- In James O'Barr's The Crow, the protagonist, possessed by a murderous vengeful spirit, resembles (though he is not) an evil clown or mime.
- In Tim Burton's The Nightmare Before Christmas, the residents of HalloweenTown include the so-called "clown with the tear-away face," a hideous, seemingly undead clown.
- In the cartoon The Powerpuff Girls, the girls meet a jolly clown in multicolor attire nicknamed Rainbow the Clown. He later gets splashed by bleach, which turns his appearance black and white, makes him insanely evil, and gives him the power to turn anything he touches into black and white, becoming Mr. Mime. Although he turns back to normal at the end of the episode, the girls still beat him up, an act which offended many viewers. Apparently he was set free, as he is seen in latter episodes in crew shots.
- A recurring villain on Teamo Supremo is The Birthday Bandit, a clown outraged with having gone through poor birthdays himself and intent on ruining other people's birthdays.
- In the movie Scary Movie 2, an evil clown attacked Ray, the gay black man. This is another parody of both the evil toy clown scene in Poltergeist and Pennywise from It.
- A clown doll in the first Ghoulies movie is possessed by Ghoulie in the end.
- In an early episode of Mighty Morphin Power Rangers, Rita Repulsa's Pineapple monster takes the form of Pineapple the clown.
- In the movie series SAW, the antagonist sends a clown-like doll on a bicycle to deliver macabre messages to his victims.
- The character of Captain Spaulding in the Rob Zombie directed films House of 1000 Corpses and The Devil's Rejects.
- Rudy the Clown, a grotesque-looking clown is the main enemy in the Game Boy Color game Wario Land 3
Movie poster for the 1989 film Clownhouse
- In the Victor Salva film Clownhouse, lunatics murder three circus clowns: Cheezo, Bippo, and Dippo, and assume their identities while terrorizing children.
- In the manga series Angel Sanctuary, the demon Belial appears in makeup and attire similar to that of a clown or mime.
- Frenchy, star of Evil Clown Comics, a recurring feature in National Lampoon magazine in the late 1980s and early 1990s, first appearing in the June 1988 issue.
- Kefka Palazzo, the main villain of the SNES game Final Fantasy VI, wears face paint and clothing reminiscent of a clown.
- Phillipe (a circus clown) is the boss of the England area in the SNES game Final Fight 2.
- The movies Xtro, S.I.C.K., and Killjoy all feature evil clowns.
- Freako the Clown is a nemisis in the Mighty Max episode 'Clown without Pity'
Examples of Clown fears:
- The animated series The Simpsons; ("Can't sleep, clown will eat me.") (Later used as the basis for an Alice Cooper song.)
- In the animated series Rugrats, Chuckie Finster is deathly afraid of clowns.
- The animated series Animaniacs; in the episode "Clown and Out" it is revealed that the characters Wakko and Thaddeus Plotz have severe cases of "clownophobia."
- In the sitcom Seinfeld, Cosmo Kramer had a recurring fear of clowns which was most prominent when the psychotic "Crazy" Joe Davola, who was after him, was wearing a clown suit as a disguise.
- The TV series Buffy the Vampire Slayer, Xander Harris is revealed to have a childhood fear of clowns in the Season one episode, "Nightmares."
- In the animated series The Grim Adventures of Billy and Mandy, Billy had a fear of clowns (which results in him continually screaming that they're going to "DES-TUH-ROY US ALL!!"), but later grew to just hate them.
- In Sex and the City, Miranda Hobbs insists that there are to be no clowns at her son Brady's first birthday party. When the baby's paternal grandmother orders one to appear anyway, Carrie Bradshaw shrieks when she sees him at the door.
- A late 20th-century witticism went as follows "Everybody loves a clown, but everybody hates a mime."
- In the TV series, Las Vegas, Ed Deline was said by Delinda to always go on a trip whenever there is a clown convention going on in the casino.
- Frasier Crane played a practical joke on his father Martin by dressing up as a clown in an episode of Frasier. This joke backfired when Martin had a minor heart attack.
- The clown doll pictured on the BBC's Test Card F has been found to be an object of fear for some 
- On an episode of Family Guy (Petergeist), Chris Griffin is cowering in his bed and says "I wish that scary clown at the end of my bed would go away!" The scene cuts to the end of his bed where Ronald McDonald is casually sitting in a chair and draws a pair of golden arches in mid-air. Chris screams.
- In the TV series Monk, the title character claims to have a fear of clowns.
- In the game Fallout 3, characters talk about a serial killer named The Pint Sized Slasher, who wears a mask similiar to that of a clowns.