THE AERO-PANTHERS.
THE INFRA-RED ANT.
THE GIANT SEA BULL.
THE QUILLED ANEMONES.
And the SEA SKATER and THE GIGANTO-WHALE..
All of these monsters are commanded by a mutant race of "watermen," led by "King Nephtoon," but because they seeks a villainous goal, that of conquering N'Yark, they rate as villains rather than monsters.
I didn't expect to find a crossover while buzzing through a 1976 HOT STUFF but here's the Little Devil saving Jack Frost from the villainous Iceman in "The Deep Freeze Mystery" (HS #137).
I devoted one post here to an installment of the silly time-travel series from JUMBO COMICS, "Stuart Taylor," in which Taylor and company went back in time to encounter the characters of Washington Irving's purely fictional story, "The Legend of Sleepy Hollow." But hey, maybe in Taylor's universe. Sleepy Hollow was real.
In the Grant Morrison ANIMAL MAN run, James Highwater is an anthropologist who helps the hero undergo a "vision quest," though he's not a standard "mystical Indian" in any way.
The cover to KRAZY KOMICS #2 is a null-crossover between Tessie the Typist and her boyfriend (who are in the comic) and Li'l Vinegar (who is not).
But Basil Wolverton contributes a real crossover between a goofy pilot-character, Flap FlipFlop, who'd been launched the previous year, and his much more popular hero Powerhouse Pepper. For good measure Wolverton tosses in another null-crossover, having Pilot Flap comment on reading a "Tessie the Typist" comic book.
Also in WILLIE #16, we get a totally unheralded crossover between the alliterative Jeanie Johnson and Millie the Model in "A Modeling Mood."