Adapting an existing manga into an anime series is likely a tricky process, regardless of whether it's a long finished series or one that's still running. Most associated with the latter to keep from catching up, though occasionally done with the former for other various reasons, is creating "filler" to help pad out the length of said anime. Very often filler is the creation of original stories & characters that never appeared in the original material, usually with highly varying results; some filler is celebrated, while others are downright despised. Sometimes, though, filler isn't original content, but rather is actually an adaptation of something that isn't the original manga.
Unsurprisingly, this is usually not known to most viewers of said anime, resulting in some thinking that it was nothing but the usual type of filler. It's not common by any means, hence why this list is "Not Quite" twelve entries long, but there are instances of anime adaptations of manga that, to varying degrees, relied on outside material for filler. Probably the most common come from light novel spin-offs, but that will be Part 2, as I was able to find exactly six examples of that. For Part 1, we'll be looking at examples where other manga that are related to the main source of adaptation are used in the anime. None of these examples can really be categorized as outright filler, either, as most of them don't take up an entire episode in length, but instead are interspersed into their respective episodes.
Ring ni Kakero REAL in Ring ni Kakero 1: Shadow
Considering whose blog this is, let's just get the obvious example out of the way first. The Ring ni Kakero 1 anime, though adapting a manga that finished up back in 1981, is no stranger to featuring content that's 100% original. The first two seasons from 2004 & 2006 in particular had numerous bits of original content, with Season 1's new stuff mainly helping set up the other major characters & the World Rivals, while Season 2/Nichibei Kessen-hen expanded on some of the fights (though Ishimatsu vs. Monster Jail was stretched out way more than necessary) & even included a shout-out to Ring ni Kakero 2 by including Cesar Juliano (one of that series' World Rivals) as a little baby. In fact, the latter two seasons, 2010's Shadow & 2011's Sekai Taikai-hen, featured no filler whatsoever, with Shadow in particular nearly being a 100% panel-for-panel adaptation of the manga's version of that story arc. Not just that, but the third season actually started everything with off by adapting the most recent piece of Ring ni Kakero 1 manga ever made.
Released at the very end of 2005, well after the first season of the anime had already aired, Ring ni Kakero REAL - Kurumada Masami Nekketsu Taidan Densetsu/The Legendary Hot-Blooded Talks with Masami Kurumada was a 200 page book that featured conversations between Kurumada & Yoshihiro Takayama, Masato, Yuji Nagata, Shinobu Kandori, Katsuhiko Nagata, Musashi, & PRIDE co-founder Nobuyuki Sakakibara, all of which were giant fans of the original RnK manga back in the day & went on to compete in promotions like PRIDE, K-1, & New Japan Pro Wrestling. Also housed within this book, though, was Ring ni Kakero 1 Gaiden: Jun to Jhun! Hikari to Kage no Maki/Put it All in the Ring 1 Side Story: Jun & Jhun! Light & Darkness, a 26-page manga short done by Kurumada. It took place right after Ryuji Takane & Ishimatsu Katori managed to rescue Kiku Takane from the Shadow Tower, with Jun Kenzaki visiting the outside of the Omura Gym before heading off to Aokigahara, which houses the main base of the Shadow Clan. There, he clashes briefly with Jhun Shadow, the leader ("Sousui", which is what nearly everyone calls him later on, anyway) of the Shadow Clan, as both a test of his own incomplete superblow Galatica Magnum & also to see how strong his own personal "shadow" had become. When the Ring ni Kakero 1 anime returned after a four-year hiatus, head writer Yosuke Kuroda seemingly decided that this side story would make the perfect start for the third season; this is literally what Episode 1 starts with. While it's not a 100% adaptation of the story, as it only covers the pre-clash conversation between Kenzaki & Sousui, it's still really awesome to see the anime adapt such an obscure part of the franchise, & the bit where Kenzaki visits the Omura Gym facade is later adapted when it actually happens near the end of Episode 3. Sometimes, it's the little things that can count the most.
Saiyuki Gaiden in Gensomaden Saiyuki
Now, yes, Saiyuki Gaiden, the far-prequel to Kazuya Minekura's Saiyuki, was indeed adapted into a three-episode OVA series in 2011, but that actually wasn't the first time said spin-off manga was actually adapted into anime. The original Gaiden manga, which detailed Son Goku's early life with the prior incarnations of his future travel buddies, debuted back in 1999, only two years after the original manga did, but due to many hiatuses wasn't properly finished until 2009. Still, having been created that long ago did mean that when the original manga was adapted into its first TV anime, 2000-2001's Gensomaden Saiyuki, the staff for that series did technically have two different manga that they could adapt from. Therefore, when the anime started relying on filler, which the anime adaptations on the whole are extremely infamous for (only about 1/3 of the TV anime episodes in total are from the manga), the staff went to the Gaiden well for as much as they could.
Now, to be fair, going off of the Saiyuki Wiki it looks like the original TV anime stuck to the original manga more often than not, at least until episode 22. From episode 23 to 50 (the end of the original series), only four episodes actually featured adapted content. Of those, only the last episode took content from the original manga (a single chapter), while all four of them in general adapted from Saiyuki Gaiden. In fact, the Gensomaden Saiyuki anime wound up adapting the first nine chapters of Gaiden, which is pretty impressive, to be honest; I do wonder how the later OVA adaptation matches up with these episodes, though. The later two seasons of the Saiyuki anime, Reload & Reload Gunlock, featured no adapting from Gaiden, but at the same time neither of those two shows were really big on adapting the main series, either. Hell, Reload barely featured stories from the Reload manga, instead focusing on finishing up the first series! Still, Kazuya Minekura's "Extreme" take on the Journey to the West story features a nice bit of lore to it, & it's cool to know that at least the original anime took advantage of that, even if only for a handful of episodes.
The Original Food Wars One-Shot in Food Wars: Shokugeki no Soma
Many manga actually made their first ever appearances in the form of one-shots, which were then popular enough to be expanded into actual serialized stories. When a manga is adapted into anime, however, that original one-shot is more or less ignored due to how it's usually notably different from the series it would later become. That being said, sometimes the one-shot wound up doing at least one thing in a more interesting fashion than how the later series would handle it, and in at least one anime it was deemed that a scene from the one-shot would be adapted in place of what the main series did. For such an example we can look at the very first episode of the first season of the Food Wars: Shokugeki no Soma TV anime.
The original Food Wars one-shot by Yuto Tsukuda & Shun Saeki was mainly focused around main character Soma Yukihira (looking notably different in some ways form his later incarnation) taking on Rossi, the main follower of Totsuki Academy's prodigy chef Erina Nakiri, in a "shokugeki" (literally "Food Fight"), because Soma dared to call Erina's cooking dull. The shokugeki would be something introduced a little later on in the main series, but the one-shot is very similar to the main series in that Soma & Erina don't start off on the right foot at all. In fact, Erina's reaction to tasting Soma's shokugeki concoction is essentially the same one as her reaction to Soma's meal in Chapter 3.
In that case, where is the one-shot adaptation that I'm bringing up? Well, it's the most subtle inclusion I'm mentioning, because it literally is only a few seconds long, & it's all about Erina's introduction. In the main series, Erina is introduced by way of a reaction she's having to soup, where she feels as if she's relaxing in a hot spring, but realizes that a giant gorilla is in there with her; an amusing reaction, no doubt, but the staff behind the anime felt that the one-shot did her introduction better. Therefore, the anime's introduction of Erina has the soup reaction of her feeling like she's meditating under a waterfall... Until a jukebox lands on her head. Much more surprising, much more exciting, & overall the better choice, to be honest. It was really the only bit of the original one-shot that could be adapted, & it's really cool to know that it was the road that was taken.
Hareluya in Hareluya II BØY
Some anime adaptations of manga are a little more "liberal" when it comes to actually adapting material. Triangle Staff's 1997 TV anime based on Haruto Umezawa's Shonen Jump hit of the 90s, Hareluya II BØY, is a bit of an example of that. The writing staff lead by the cult-favorite Yasuhiro Imagawa essentially ran the entire gamut when it came to the varying levels of adaptation. The BØY anime featured episodes based directly off of the manga, it featured anime-original stories, & it even had episodes that combined manga stories together in some ways, or changed the way some stories played out while still being technically adaptations. One of the most interesting alterations made, though, was how the anime featured some slight hints of the original version of Umezawa's lead character Hareluya Hibino, which was seen in the single-volume Hareluya, which ran during 1992. In that original take, Hareluya was the literal "Son of God" who had been cast down to Earth in order to learn humility. BØY's reboot of Hareluya had all of the same ego & bravado as his original inspiration, but was simply a human boy; a boy with seemingly inhuman stamina, mind you, but still definitively human. The anime, however, was much more vague about it.
*Technically, I'll be giving some spoilers regarding the anime... But this is such an inaccessible show right now (no DVD release or English fansubs, natch) that it likely doesn't matter.*
It all starts with the avant (i.e. pre-opening scene) in episode 1, which shows the "Son of God" Hareluya being flung through space before falling towards Earth. This scene is never brought up again, though Hareluya's habit of pulling weapons out from behind his back from BØY now featured a bright light every single time (not to mention went far beyond simply baseball bats & fry pans). Finally, though, the last episode of the anime started off with the same exact avant as episode 1, but this time featured an actual adaptation of the first chapter of Hareluya... Well, okay, it was only a few pages from the chapter, detailing Sister Chris' altercation with some local punks before that manga's Hareluya crashes down onto the punks' car. Still, seeing original Hareluya adapted into the Hareluya II BØY anime was curious enough, but what came next was even more interesting. Right before the OP played, the anime would reveal that the boy that fell to Earth was not Hareluya, bur rather was his father Harebare, which would make the BØY anime's Hareluya Hibino the "Son of God" that he was originally conceived as by Umezawa!
However, the episode was previewed by Hareluya with the alternate title "Hareluya on the Dream", & the stinger after the end credits revealed that at least some part of the episode (particularly the completely bonkers finale) was all a dream that Hareluya was having. Therefore, Imagawa & crew purposefully left Hareluya's status as God's literal child up to personal interpretation, which I think was genius. Was the entire episode just a daydream Hareluya was having while sleeping during class, or was it just the final battle between him & (obvious Devil counterpart) Kanzaki that was the dream? The choice is yours!
Sadly, these are the only four instances where anime adapted another manga directly that I could find, so as a way to get prepped for Part 2's list of light novel uses, here are two light novels spin-offs that could have been adapted & would have fit this list, but were not... At least, not yet.
Saint Seiya: Gigantomachia
Quite frankly, it's kind of astonishing that this has not yet been adapted into anime by Toei Animation, especially considering how the studio has produced a Saint Seiya anime nearly every single year since 2002/2003 (minus 2009-2011, which had TMS' Lost Canvas OVAs & the last two seasons of Ring ni Kakero 1). Granted, I'm leaving out manga spin-offs Episode.G & Santia Sho, but this specific spin-off predates every single one of those. Saint Seiya: Gigantomachia was a two-volume novel written by Tatsuya Hamazaki, who has written for a variety of franchises, like the One Piece novels, Liar Game novels, .hack (both Legend of the Twilight & //G.U.), & some Digimon spin-offs. Coming out in 2002, with the second book coming out literally days before Episode.G debuted, Masami Kurumada is credited as a supervisor for the novel, & it's actually the first time any new Seiya story had even been produced since the manga's end in 1990.
Taking place between the Poseidon Chapter & Hades Chapter, Gigantomachia has Seiya & his Bronze Saint brethren take on a group called Gigas, who represent the Giants of Greek myth, in a modern day take on the Gigantomachy. Working with Seiya's group are three new Saints: Bronze Saints Coma Berenices Mei, a student of Cancer Deathmask, & Sextans Yuri, plus Silver Saint Ara Nicol. Sadly, there's next to no information on Gigantomachia when it comes to English, since there is no fan translation at all, & even the Saint Seiya Wiki lacks any page dedicated to the novel. If anything, that right there makes it a perfect choice to adapt into an anime of some sort, but that could very well never happen. First of all, the novel apparently isn't considered canon to the Seiya timeline, being deemed an alternate universe story, though one could easily argue the canonicity of stuff like the TV anime's Asgard Chapter & last year's Soul of Gold (which relies on the former for some story).
The second point, however, is simply the fact that there is so much more Saint Seiya out there to choose from now. Again, if I was betting man then I'd be putting my money down on the next thing from Toei being the long awaited Next Dimension anime, since it would be a perfect way to celebrate Seiya's 30th Anniversary this year (& ND's 10th, too), & Kurumada is apparently going to finish it up sooner rather than later. Simply put, Saint Seiya: Gigantomachia, with it's non-canon status & relative obscurity, would be a perfect choice for this list had it actually happened, but who knows if it ever will be so.
The JoJo's Bizarre Adventure Light Novel
As a complete reverse of what Gigantomachia is, this is an example of something that could have been adapted into an existing adaptation, & it would have been really neat to see happen. Hirohiko Araki's JoJo's Bizarre Adventure has such an interesting concept by being comprised of different "Parts", which can technically allow for a variety of light novel spin-offs, since there are so many different stories to work off of. Bizarrely enough (pun most certainly intended), light novels didn't really become a thing for this franchise until the new millennium, minus for one, & even then there aren't too many. In total there are only six JoJo novels, with the last four having been created in the past decade alone. Before that was a Part 5 novel in 2001, but the one I'm focusing on here is the very first JoJo novel, released way back in late 1993.
Simply titled JoJo's Bizarre Adventure (though it was given the subtitle The Genesis of Universe when released in Italy in 2003), the novel was written by Hiroshi Yamaguchi (Desert Punk TV) & Mayori Sekijima (The SoulTaker), featured artwork by Araki himself (which is something he did for all of the novels), & told two original stories that took place during Part 3: Stardust Crusaders. The first story, One-Way Trip from Desert to Hell, takes place shortly after the fight with N'Doul (the first of the Egyptian God stand users), & has Jotaro & his friends decide to risk taking a locomotive to escape the Egyptian desert & reach Aswan. Not surprisingly, the train is actually the stand Satanic Coupler, which is wielded by Absalom, & also on board is Absalom's sister Michal, who wields the stand Dark Mirage. The second story, The Gravestone of Red-Hot Sand, takes place after the confrontation with the pairing of Hol Horse & Boingo/Mondatta, though the apparent lack of Iggy indicates that it happens during his (animal) fight with Pet Shop. Here, Polnareff, Joseph, & Avdol get attacked by a mummy-like old man named The Scribe Ani, who wields the stand The Genesis of Universe, which can summon the historical events written in its pages.
Considering that Araki drew artwork for these two stories, it can be argued that they are possibly considered canon to the Stardust Crusaders story. Not just that, but the TV adaptation of this story arc totaled only 48 episodes, which isn't exactly odd but it does make one wonder why David Production didn't simply adapt these two novel-only stories & make the entire thing a solid 52 episodes (if both would have even needed two episodes each). Obviously, since both take place after the gang reaches Egypt, it would have made the second season of this anime (Egypt Arc) longer than the first season, but at the very least it would have given something new for most viewers. Considering that this novel only saw release in Japan & Italy, most international JoJo fans have never read these two stories, & having them be adapted into the anime would have been something brand new for them, let alone those who never experienced this story arc in the first place.
Who knows, maybe David Pro will one day create OVAs for not just this novel, but also the other five should the JoJo anime maintain the popularity & success it has managed to maintain since 2012. I've heard that the Jorge Joestar novel from 2012 is possibly the most bizarre tale in JoJo history, which REALLY says something, but that story requires knowledge of how Part 6 ends, so that won't happen for a while.
Such is the end of Part 1 of this list of when anime goes above & beyond the manga for adaptation material. Going off of those last two "possible in the future" choices, Part 2 will be all about those moments when anime looked at light novel spin-offs of manga for content, because that actually happens more than you might think.