Manga serials are not known for crossing over with other manga serials maintained by other creators, but here's a couple where it's happened recently.
NISEKOI did a two-part crossover with a less well known manga, OREMONOGATARI X. pretty forgettable.
A more lively crossover is one in which the popular NAGATORO literally collides with another lesser known, TEJINA SEMPAI.
Here's yet another largely unexplained convocation of the "Horrific Trio" in an episode of SPIDER MAN AND HIS AMAZING FRIENDS. In it Dracula comes to America, hypnotizes Firestar and takes her back to Transylvania, where he's served by a wolfman and a sort of "Frankenstein robot." Iceman and Spider-Man follow to rescue the maiden from the monsters.
In issue #83 of THE MARVEL FAMILY, the three Marvels contend with the ghost of ancient Indian Chief Thundercloud, who actually manifests as-- a big old cloud with a feather-headdress.
"Palo" was, for the first two issues of KEY COMICS, the sidekick of a white-guy adventurer named Jeffrey Quick. The title of the feature was "The Key" not because that was the name of the main hero, but because Quick carried around a heavy key on a chain, with which he, uh-- fought crime? I think something was said about Palo's tribe giving Quick custody of the possibly magical key, though it didn't actually do anything. Quick seemed to think he could sense crooks with the aid of the key, and he could throw it so that the chain looped around the neck of an adversary, and-- stopped him cold? The most one can say about Palo is that his broken English is overshadowed by the sheer dumbness of this concept. Apparently the editors realized after two issues what a dopey idea this was, since for the last three issues of KEY COMICS, Palo disappeared and Quick just narrated weird stories that involved keys. A textbook example of a publisher trying to come up with a novel idea but not thinking about whether or not it had any intrinsic appeal, even to undiscriminating kids.
And here we have another Japanese comedy about a harem of sexy monster-girls, MONSTER GIRL DOCTOR, which includes a wide variety of monster-girls constantly pursuing a handsome but mostly oblivious human doctor, reviewed here.
I've finished a review of 2015's FRANKENSTEIN VS. THE MUMMY. Since the two creatures are co-equal figures in the narrative, this might also stand as a crossover-- more on which elsewhere, perhaps.
Addendum: though I may never get around to putting the rationale into words, I would now regard this as a "null-crossover," a term coined on ARCHETYPAL ARCHIVE, which applies to a film where one established character crosses over with one only appears once but does not generate his own "character cosmos." This stands in contrast to "spin-off" characters who may have no history when they first appear, tied to some established character, but who do go on to generate such a history, even if only through just one repeat appearance.
