I fanatically consumed all of Douman(Dowman) Sayman´s work recently.
Especially the lighthearted horror atmosphere and the great artstyle he creates, is what i enjoyed most.

Now I am on the hunt for more that could be comparable to this theme.

Takahashi Yousuke's manga, especially the ones tagged with comedy and horror/supernatural.

Abe Youichi's works.

Though they're more action-centric than the above examples, you might also want to try Jabberwocky and its sequel. Likewise, other works by Hisa Masato.

Yeah, I can't swear to any of the rest of TAKAHASHI Yousuke stuff, but certainly Mononoke Soushi should be similar.......I think I'd also suggest Choku!...... while the style isn't the same, (I mean, you know, this is comparing it to freaking Douman Seiman, you're just not going to find a lot that really reaches those levels of bizarreness) the humor is quite offbeat and I think it somewhat shares the "quirky characters" aspect.

Good luck 🙂

Oh jeez, this is tough.

Thirding Takahashi Yousuke's work. They're pretty great.

Also, not horror, but very weird in the same vein, Love Sync Dream and its spinoff Orz might interest you. (If you enjoyed these two and are interested in more of these weird/off-the-wall manga, I have a few more recs for you.)

Thanks a lot for all the fantastic response.

Takahashi Yousuke´s manga look quite interesting. When even the Hellsing/Drifters creator is praising him, then it looks like a must read for me.

Kouta Hirano called him "a treasure to the manga world."

I can´t make my mind up about Choku!, maybe not exactly what i looked for but i am probably still going to read it. Because i do agree that the characters share some of the "quirky" aspects .

From what i´ve seen, ABE Youichi is definitely hitting the mark for me. Great art style and this unnameable weirdness, which i loved so much about Douman Sayman, hooked me up. 100% on my list!

Hisa Masato´s work is great , i already read Nobunaga(u)n and Tsukinowaguma Koroshiya. So Area 51 and Jabberwocky won't be left out. 🙂

Also just read Orz today, it was quite nostalgic and i really had a good time.

If you enjoyed these two and are interested in more of these weird/off-the-wall manga, I have a few more recs for you. On point, feel free to list more.

Overall you guys hit the mark quite well and i really really appreciate all recommendations.

Ashizuri Suizokukan Very surreal. Has more in common with alternative comics than manga.
Dainana Joshikai Houkou In general, Tsubana writes a bunch of interesting off-the-wall stuff. This one is probably one of her weirdest that's translated, but the shorter Mikake no Nijuusei is good too.
Hikidashi ni Terrarium
Ryuu no Kawaii Nanatsu no Ko
Sneewittchen - Geschichten nach Grimm

Kyou Machiko is worth checking out too, especially her Mitsuami no Kami-sama. She's kind of the opposite of Dowman, though, in that her works seem light-hearted because of the style but the underlying tone/message is kind of horror-esque.

Seconding Suxinn's new recs, which I think are fairly spot-on.

Another mangaka along these same lines is Murai, especially his Rojiura Daiikku and Sakai no Musume - Cat in the Car.

Henshin no News and Sakura no Sono (FUMI Fumiko) are both worth checking out.

Another "weird" mangaka is Kuroda Iou, in particular Sexy Voice and Robo, Nasu and Nainippon Tengu Toukaishi. But the hitch is that while his stories and characters are plenty weird, the underlying message is serious, realistic and down-to-earth. I've never really found any of Seiman's stories to be philosophical, so Kuroda Iou might not resonate with you. Still, thought to mention it as a just-in-case.

Thanks again , i will have myself worked through them all eventually.

It is actually surprising how much good alternative work there is.

For comedic horror, you could try Shintaro Kago's works, although they are quite graphic. If you haven't read anything by him, start with Harem End or The 'Can an Accidental Collision on the Way to School Result in a Kiss' Experiment.

I'd also recommend Ranman.