Halloween Cosplay

Fan Art is a common and understandable pastime. But drawing a fictional character dressed as another fictional character? This is actually common too. See Cosplay Fan Art.

This is a Sub-Trope where Web Comic characters will frequently cosplay for Halloween rather than buying or making generic costumes. The characters commonly appear on a single panel, which usually occurs out of continuity and can happen even if the characters come from a setting without a Halloween holiday. The characters can all cosplay to a single medium and/or cosplay as characters that inspired/influenced their own design.

Examples of Halloween Cosplay include:

Anime and Manga

During a festival in the anime film Otaku no Video, the members of the anime club show up dressed as characters from other anime.
Sayonara, Zetsubou-sensei had a segment about this.

Fan Works

Raised to a genre all its own in Buffy Fanfic. The second season Halloween episode featured a spell transforming everyone in town into their costume. A whole legion of fics, known to the fandom as "Yet Another Halloween Fics", or simply, YAHF take a variation of the episode as their jumping off point. Xander, and occasionally other characters as well, select the costume of a trademarked character. Chaos inevitably ensues.

Live-Action TV

In the Castle Halloween episode "Vampire Weekend," the title character, played by Nathan Fillion, steps back into one of his actor's old roles for Halloween.

Newspaper Comics

One FoxTrot strip (which can be seen here) showed Eileen, Eugene, Phobe, Marcus, and Jason as Violet, Spock, Hobbes, Mega Man, and Jabba the Hut, respectively.

Video Games

Used as Downloadable Content in the original Sonic Adventure 2, but not its Game Cube remake.
In the Halloween Town levels of Kingdom Hearts, Sora gets a unique costume complete with Cute Little Fangs.

MMORPGs

Gaia is quite fond of this for it's Halloween events, and the art changes every year. (With the exception of the past few years, where everyone recycled their costumes). It started out in 2004, with Agatha as Slimer, Meridith as Darth Vader, and Moria as Freddy Krueger. Other famous examples include Edmund as James Bond, Liam as Hannibal Lecter, Sasha as Samara, Ian as Jigen, Vanessa as Elvira, and Agatha as the most terrifying thing you've ever seen in your life.... Saggy... naga...boobs... X_X
- While the majority of shopkeepers didn't indulge in this trope for Halloween 2009, it did bring us Edmund as Batman. In the midst of the event, the Dernier* Cri crew returned with Brennivin as Andy Warhol, Kanoko as fellow RIG character Hatsya, and... Radio Jack Sparrow.
- Up to Eleven in 2010, with Nicolae as Alex, Moria as The Major, Kanoko as Hermione, Radio Jack as Pikachu, Ian as Goku, Rina as Chibitalia, Becky as Lady Gaga, Peyo as Max, Louie as Uryu Ishida... and the list goes on from there.

Web Animation

A staple of Homestar Runner Halloween toons.

Homestar: Hey, Stwong Bad. I weally like youw costume. The Spanish Inquisition? That's gweat!
Strong Bad: Look, for the last time, I'm not the Spanish Inquisition, I'm not Cab Calloway, and I'm not Strong Bad wearing a yellow turtleneck! I'M CARMEN FREAKING SANDIEGO!!!!
Marzipan: I like Cab Calloway...

Web Comics

The Chapel Chronicles has a special Halloween strip, Chapel tried on a Gorgeous Period Dress, the Tardis from Doctor Who, finally deciding to wear the outfit of Cardcaptor Sakura with her hedgehog dressed as Kero-chan.
RPG World is widely acknowledged as one of the originators of this trope.
In Narbonic, one of the characters cosplayed as a different character from Narbonic itself.
Seems to be something of a ritual among the 'adults' in the first part of Coga Suro, to the point that they all seem to do it even without conferring with one another.
Tom Siddell has drawn Gunnerkrigg Court Halloween pictures, with Annie and Kat dressed as characters from Titus Andronicus, The City of Lost Children, Psychonauts, and Ico.
In Bob and George, the characters would frequently dress up as other videogame characters. Interestingly, sometimes events from the Halloween party strips (and from other holidays) would affect the main Bob and George continuity, and sometimes they didn't. There was also a Running Gag involving the Author, as he would dress up as a character nobody else would recognize (like Q or the Spectre).
Cinema Bums has the main cast dress as Bill S. Preson Esq, an anthropomorphic personification of the alienating effects of modernization and a Steampunk version of Marion Ravenwood in this strip.
Exterminatus Now does this every Halloween, though they subverted it one year by having trick-or-treaters in various popculture costumes and the four main characters doing nothing special. Previous years have seen them do characters from Eight Bit Theater and various comic books.
- This year's had four trick-or-treaters dress up as the main characters.
DMFA had a New Look Montage where Abel has Dan and himself dressed up as characters from various SF/Fantasy series. One, Two, three, Four and Five
Mac Hall had a strip where the cast dressed as characters from other webcomics. Helen, who is dressed as Cherry from RPG World, comments that she has no idea what's going on; her boyfriend Micah tells her not to worry, since she's still in-character.
Kismetropolis characters dressed as the cast of Michael Jackson's Thriller for Halloween once.
Adventurers!! every Halloween. Most disturbingly, Killer Evil Death Spybot 5000 was dressed up as a Final Fantasy white mage. Yeah...
Megatokyo, here, as well as various costumes worn by Erika and Piro in their function as employees at the store.
Dominic and Luna (and sometimes a few other characters) from Dominic Deegan dress up as characters from other comics, or other characters from their own comic.
Something*Positive, numerous times, most recently when Davan dressed up his not-son as dead Robin Jason Todd with the crowbar still attached.
The comic Jayden and Crusader has taken up this trope, although originally used more generic costumes.
- That is until 2009 when two characters come dressed in Trek uniforms...
El Goonish Shive does this here and here (2002 2-parter) and here (2003). Both Elliot and Ellen dressing as Saotome Ranma is particularly fitting.
Okashina Okashi does this every year, with decently wide variation in how well-known the characters are.
Ansem Retort did this. Axel costumed himself as Zexion. Zexion costumed himself as Axel cosplaying Vash the Stampede.
Unwinder's Tall Comics plays with this rather vigorously. On Halloween 2008, Unwinder mocks three of his friends for all dressing up as Heath Ledger's Joker—while he himself goes as a fully-nude Dr. Manhattan. The following year, Barbecue Sauce cosplays Jaded Lament from in-universe anime Tokyo Delta Jetlag D for a Harvest Festival. For Halloween 2011, there's a trick-or-treater dressed as Wonderella, and Unwinder (dressed as Mark Trail) mocks Barbecue Sauce (dressed as Bender) for referring to their costumes as "cosplay".

Unwinder: Look around you. Does this look like a "con"? This is just "dressing up". It's something you can do without being a nerd. We don't need a special word for it, because it's not sixty percent of our lifestyle.

This Fanboys comic, featuring Paul as Doctor Horrible, Sylvia as Viral, and Lemmy as The Mysterious Mr. L.
The Halloween story in Skin Deep features all the main characters dressed in costume, including the sphinx character dressed as The Riddler
Dominic Deegan had one in its first year. But since that was a good eight years ago just about all the webcomic characters their outfits referenced to have long since died and vanished from the internet.
Far Out There likes doing this as one of its many holiday bonuses.
Wayfarer's Moon does this not once, but twice and throws in some Thanksgiving and Christmas for good measure.
Ruby's World has done this twice in homage to other comic/cartoon characters, 2009 and 2010.
Sancta Terra had two of the main characters dress up as each other.
Game Destroyers has had two Halloween strips, one in 2008 and one in 2009.
Terenya from Uncreation was dressed as a nurse from Silent Hill in 2010, as seen here. Fitting, since she's The Medic.
Five Color Control had its characters cosplay in this Filler Strip.
This early Brawl in the Family strip had Adeleine as Lisa Simpson, Meta Knight as Black Mage, and King Dedede as Eric Cartman.
The Law of Purple: Lette dresses up as Priest Set(o)
Stubble Trouble does this each year.
Unintentionally Pretentious has Mia and Luthor as Marge and Homer, while Sydney dressed as Akane Tendo.

Western Animation

In the Transformers Animated episode "Along Came A Spider", which took place on Halloween, Sari dressed up as Optimus Prime, and her square costumes ends up making her look more like G1 Prime.
Gargoyles, twice, once in a TV episode and later again in a comic book issue. In the TV episode, Eliza dressed as Belle from Beauty and the Beast. Her date was Goliath.