Most anime that come out these days tend to be adaptations of manga series. That isn't to say that there's never been an anime purely made as a new IP, Kiznaiver by Trigger or Carole & Tuesday animated by Bones are both series that fit that mold.
But, for the most part, anime serves as a way to bring manga illustrations to life, and they tend to wait to adapt a series until at least a couple of seasons' worth of content has been illustrated. Now, a lot of the time, the anime will catch up to the manga, but it's actually pretty common for the anime to get further and further behind. Here are some examples people might not expect of anime series with manga sources that are way ahead in the narrative.
10 Demon Slayer
Obviously, Demon Slayer had to be mentioned, considering how well its adaptation did (we all saw that Dance of The Fire God clip/episode) and now we've got the Infinity Train movie coming soon. But, how many anime-only fans know that the series has actually already been finished for a while now? That's right, back in May of 2020, the final chapter of Demon Slayer released in Shonen Jump and the series came to a close. Frankly, the anime still has a ton to cover, with the Infinity Train story probably sitting somewhere at about 30-50% of the way through all its story arcs.
9 My Hero Academia
Next up is My Hero Academia, an anime adaptation that has not been slow by any means. But, MHA creator Kohei Hoshikori has just been so consistent with his releases and there have been 1+ year-long breaks between each season that the manga is actually way far ahead. Slight spoiler warning, but anime-only fans who hang around Reddit and forums very likely know all the secrets of One for All by now, but that part of the anime series is actually only coming out in Season 5. Honestly, arcs like the My Villain Academia event or the current one with The Lab Raid are probably not coming for another season or two at least.
8 Jujutsu Kaisen
Jujutsu Kaisen just started releasing on Crunchyroll and is only on episode 4 at the time of writing this, so it's safe to say this is likely the entry on this list the farthest from its manga source. The current chapter count of the Kaisen manga is at 127 chapters as of Oct 25th and the first couple episodes only cover maybe 9 chapters at most. So, there's a ton of content left to adapt and much more we'll be seeing from Yuji and friends, so here's hoping animation studio Mappa does an amazing job with it like it seems like they've been doing so far.
7 Hajime No Ippo
The 3rd Season of Hajime No Ippo first came out back in late 2013 and there's been very little news since about a new season or adaptation attempt. But, people who just stumbled upon the series likely don't know that it's been in publication for over 30 years now, with over 1300 chapters (that's way more than One Piece by the way).
So, it's safe to say that anime is way far behind the manga, which actually looked like it was finally reaching its conclusion earlier this year, but is still somehow soldiering on.
6 Vinland Saga
The story of Thorfinn and "friends" in Vinland Saga is a deceptively long one and is commonly put up there on a pedastal with the greats like Berserk or Vagabond. And, the amazing anime adaptation that came out on Amazon Prime Video in mid-2019 only scratched the surface, truly. Fans might be surprised if a Season 2 of this series comes out, as the story arc after the one where the anime ended is a lot more calming and is the first part of Vinland Saga's major shift in tone. But, if readers/viewers make it through the (frankly amazing) lull of this arc, they'll find that there's just so much story left to cover and adapt.
5 The Way Of The Househusband
Way of the Househusband is a series coming out on Netflix in 2021 and we couldn't be more excited. It's a gag manga about an infamous Ex-Yakuza Boss called The Immortal Tatsu who settles down with the love of his life and learns to become a house husband. But, it's hard to take the Yakuza attitude out of the man, so he still acts like every matter is life-or-death Yakuza business, even when folding laundry or making a bento-box. Manga fans, in general, have likely seen hilarious screencaps of Househusband chapters, and we're so excited to see the anime and real-life adaptation of this series, though there's more content than people might be aware of, with four volumes worth of material already published.
4 The Promised Neverland
When fans last left off on the first season of Emma's horrifying adventures in The Promised Neverland, most of the children finally managed to escape Grace Field. But, what if we told you that was basically the first fifth of the story, and it actually already reached its conclusion in the manga, much like Demon Slayer did earlier this year? Yes, The Promised Neverland anime has more potential content to cover than Demon Slayer does, and things change so much between what anime-only people have seen and what the manga-fans know. We can't wait for it to get adapted (if it ever does) or how they'll animate certain sequences of the story.
3 The Rising Of The Shield Hero
Out of all the stories in the Isekai genre, The Rising of the Shield Hero is one of the ones most beloved by fans. This series tells the story of a guy who is so immediately screwed over by those around him and yet somehow finds a way to make a life for himself as a summoned hero in a different world.
It's a very good story, but the manga is a ways away from where the anime ended. Luckily, it's confirmed that a Season 2 is coming, but we'd guess that they wouldn't be caught up to where the manga is at until at least Season 3.
2 That Time I Got Reincarnated As A Slime
Here's another Isekai entry that is so surprisingly good (and constantly gets compared to Shield Hero) that people can't help but watch. This tale follows the exploits of a man who was reincarnated as a slime who eventually dawns the name Rimuru Tempest. Rimuru is one of the most likable MC characters we've seen in years and he just seems so able to solve every problem that comes his way while still seeming empathetic and understanding. Of course, it may not still be that way in the manga, as it's at least a few arcs ahead of where the anime ended and has plenty of new characters and conflicts to introduce, one of which is so surprisingly dark we almost don't want it to be adapted.
1 Attack On Titan
And lastly, there's Attack on Titan, a series that apparently will be ending (at least the anime) on December 7th, 2020. But, that's sort of odd, considering that the story isn't over yet in the manga, and it's at least a season and a half away from the anime in terms of content to adapt. So, this might be a situation where the anime team comes up with a new ending while working with the creator, or it may be a two-cour season that just barely manages to cram everything in. But regardless, Attack on Titan the anime is way further behind the manga than the new team at Mappa (now taking over for Wit Studios) is leading us to believe.