The 1970s was a bit of a "lawless" time in the world of comic books, so unregulated in fact that Mantis of the Guardians of the Galaxy was actually written into comics under 4 different publishers by her creator Steve Englehart. Mantis, or the "Celestial Madonna," has always been much more cosmically important in Marvel Comics than in the MCU, so important that Englehart brought her into DC, Eclipse, and Image Comics!
Raised in a Kree-worshiping religious sect called The Priests of Pama, the martial artist and superheroine Mantis debuted in 1973's The Avengers #112, introduced as a half-Vietnamese, half-German woman with incredible fighting skills and a tendency to refer to herself in the third person. Originally created by Steve Englehart, known for legendary runs on Green Lantern, The Avengers, and Doctor Strange, the author used Mantis as a major character throughout his run on The Avengers, and loved the character so much that he decided even Marvel Comics couldn't contain her.
While the green martial artist characters Englehart placed into future comics never explicitly call themselves Mantis, he has officially confirmed that it was his intention that they were all intended to follow Mantis along her journey of motherhood. The end of Englehart's tenure on The Avengers concluded with the massive "Celestial Madonna" story, framing Mantis as the destined mother of the "Celestial Messiah," sired with the Prime Cotati, a plant-life being with long-held ties to the Kree and Skrulls. Mantis' story ended with her going off with the Prime Cotati who was inhabiting the body of her ex-Avengers teammate the Swordsman to become pregnant and bear her child among the stars. With Englehart departing soon after, he apparently decided that he wanted to continue the "Celestial Madonna/Messiah" story, which ended decades later in Empyre, leading to the character of "Willow" appearing in DC Comics Justice League of America #142 in 1977. Willow was a green-skinned woman with incredible fighting skills, celestial knowledge, and a penchant for referring to herself in the third person, telling Aquaman, "This-one has come from a place she must not name, to reach a place no man must know!"
This is clearly a not-so-subtle hint by Englehart that Willow is in fact Mantis, and can't say her real name or explain her plot in detail because of copyright issues, and by the end of the issue, Willow is already leaving the JLA. She tells the Atom, "People change! Times change! Names and bodies Change!" Willow then goes on to pretty explicitly explain how she went from being a normal Earth human to meeting a cosmic entity who she went away with, and eventually got pregnant by, and with that story Willow left the DC Universe and has never returned. Several years later Mantis turned up as Lorelei in Englehart's Scorpio Rose series at Eclipse Comics, a reworking of a Madame Xanadu series for DC that was canceled, and she has now given birth to her child (later revealed to be Quoi the "Celestial Messiah") and is raising him in a suburban community. Lorelei was then seen again in Coyote Collection #1 from Image comics several years later, again written by Englehart, ending Mantis' multiversal journey throughout the world of comic books. The fact that Englehart was able to so successfully weave Mantis' "Celestial Madonna" story throughout several different comics publishers is endlessly fascinating, and also goes to show how important and versatile the underutilized character Mantis is.
While in modern times it is almost a 100% guarantee that Marvel Comics would never allow a writer to casually bring their creation over to DC comics, it is an awesome piece of comic history that decades ago the legendary Steve Englehart transported Mantis, of Guardians of the Galaxy fame, through the worlds of DC, Eclipse, and Image Comics.