[[AC:From the Series as a whole:]]
* RecursiveAdaptation: ''Hellboy Animated'' has its own comic books.
* WhatCouldHaveBeen: There were actually plans in the 90s to create an animated TV series with ''Creator/{{Disney}}'' of all things. While this never went anywhere a lot of the concepts from the pitch wound up mutating into what eventually became ''WesternAnimation/{{Gargoyles}}''.
* WriteWhoYouKnow: Hellboy himself is largely based on Mike Mignola's father, who was a cabinet maker. He'd often come home to his family with tales of horrific on-the-job accidents, including one careless fellow losing a hand, told in the nonchalant, unflappable manner that would become HB's trademark.

[[AC:From the Comics:]]
* AuthorExistenceFailure: Industry legend John Severin was asked what types of story he'd like to draw when he came onboard. His first request, to do a [[TheWestern Western]], led to the ''Witchfinder'' story "Lost and Gone Forever". His second request, to do a WWII story, led to the ''Sledgehammer '44'' series being developed, but he died after only drawing the first three pages and some character designs. The series was eventually completed by other artists.
* ExiledFromContinuity: The appearance of the Torch of Liberty in the first miniseries is still canon, but since the character is created and owned by John Byrne, who scripted the original miniseries, he has never reappeared outside of Hellboy's "family photo".
* WritingByTheSeatOfYourPants: Mignola plots the series ''years'' in advance, but he did change his plans for ''Hellboy In Hell'' which was originally intended to run at its own pace indefinitely. But after [[spoiler: Hellboy killed Satan]] too many dominos were pushed over and he decided to go for a GrandFinale with issue ten instead.

[[AC:From [[Film/{{Hellboy}} the films]]:]]
* AwesomeDearBoy: Ron Perlman signed up because he loved the idea of doing a BeastAndBeauty love story [[ActorAllusion again]], and he would get to kiss a beautiful woman half his age in the process.
* ChannelHop: A rare film example; switched from Sony Pictures to Universal. This was due to Revolution, who produced the first film and had an exclusive deal with Sony, had folded at the time.
* DawsonCasting: A weird case, as Hellboy is technically older than Ron Perlman, who plays him. Except that Hellboy is supposed to be OlderThanHeLooks and in his 30's appearance wise. Ron Perlman was in his 50's when he played the character, meaning he was a [[MindScrew man in his 50s playing a man in his 60s who looks like a man in his 30s.]]
* DevelopmentHell: The third film has become this and chances of it out is now unlikely as in an early 2017, del Toro confirmed that it's 100% cancelled.
** It has been announced that a reboot is scheduled to come out some time in 2018.
* ExecutiveMeddling: Executives originally wanted to cast Creator/VinDiesel as Hellboy and make him a human who can transform into a demon, a la TheIncredibleHulk. Putting aside the fact that this would most likely have gotten them sued by Marvel Comics, del Toro and Mignola did not like this, and they had both independently decided that Creator/RonPerlman was the only casting choice for the part that either of them would support. Hellboy remained a big red guy in a coat, and Perlman was cast. The film took a significant budget cut as a result, though given del Toro's talented directing, it's hard to tell- although it also meant the advertising budget was almost nothing, and since the film was released when ''Passion Of The Christ'' came out and many theaters refused to show the film due to that fact, as well as having the word "Hell" in the title, having no ad budget REALLY hurt this movie.
* MetaCasting / InkSuitActor: When discussing the film, Mignola and del Toro were considering who should play Hellboy. They each said they had a particular actor in mind and decided to each write their choice down on a napkin and turn them both over at the same time. They had both written "Creator/RonPerlman."
* TheOtherDarrin: In ''The Golden Army'', Abe is now voiced by his actor, Doug Jones, rather than David Hyde Pierce. Pierce felt Jones' performance was strong enough that there was simply no ''need'' to hire a separate voice actor. The difference was practically unnoticeable, since Pierce had only been cast in the first place [[ExecutiveMeddling because some executives thought Jones wasn't a big enough "name"]], and he'd been mimicking Jones' performance anyway.
* RealLifeRelative: Blake Perlman (Ron's daughter) can be seen in the sequel as the reporter in the gray sweater interviewing Hellboy outside the auction house.
* WhatCouldHaveBeen:
** In the [[AMinorKidroduction Minor Kidroduction]] of the first film, Baby Hellboy was going to be an animatronic, but they didn't like how it looked, so they went with CG instead.
** Charlie Hunnam was actually del Toro's first choice to play Nuada. However, Nuada's design looked too extreme on him so Luke Goss was cast instead. Hunnam would later work with del Toro in ''Film/PacificRim''.
** ''The Golden Army'' originally ended with a SequelHook involving [[spoiler:Roderick Zinco (from the comics) obtaining Kroenen's severed head, traveling to a secret Nazi safehouse in the Arctic, attaching it to a massive robot body, and Rasputin's ghost manifesting before them. It was included as a motion comic on the Golden Army DVD.]]
** ''Hellboy III'', which has sadly been confirmed dead by del Toro in a February 2017 interview after a long time in development hell.