In 2008 I devoted some space here to some of the earliest examples of "monster rallies" and "villain rallies" in popular fiction. The earliest dated use of the term "monster rally" that I've found appears in a 1950 Charles Addams cartoon, but I suspect Addams didn't conceive the term. It sounds like something that would have been cooked up in the 1940s in response to Universal Pictures' release of "monster crossovers," possibly starting with 1943's FRANKENSTEIN MEETS THE WOLF MAN. If "monster rally" does indeed date back to the 1943 film, it indicates that "rally" did not connote unity for the person who coined the phrase, even though the word traces from a French word meaning "to unite." As all horror-fans should know, the film centers upon the two monsters not just meeting, but eventually coming into conflict. The next three Universal crossovers also did not depict the monsters as part of any united front: in HOUSE OF FRANKENSTEIN and HOUSE OF DRACULA, they cross paths to some extent but largely don't affect one another's arcs, In ABBOTT AND COSTELLO MEET FRANKENSTEIN, the Monster is more or less the thrall of Dracula, and the Wolf Man opposes the vampire's plans, though the lines of battle are not as strongly drawn as they were in the 1943 film.
In my essay THE LOGIC AND APPEAL OF CROSSOVERS I stated that I thought much of the appeal was about the audience taking pleasure in the differences between the respective mythologies of two or more focal presences:
Some Marxist critics will view such character-crossovers as one of many strategies by which the evil Masters of Mass Culture manipulate their audiences. While such explanations may seem to answer all questions as to the motives of the stories' producers, they don't say anything substantive about why the audiences choose to patronize not just works of mass culture in general, but works in which characters or concepts from different storylines happen to intersect. The usual Marxist explanation is that these audiences want nothing more than mindless divertissement. However, the overlapping of distinct storylines would seem to intensify the degree of mental effort an audience-member must exert in order to participate in the crossover's intersecting universes. For instance, when Rider Haggard takes a character who exists in a moderately realistic universe, i.e., Allan Quatermain, and causes him to encounter a character whose nature is overtly supernatural, Haggard must find some way to treat both characters with integrity, even though the ground rules of their universes are in conflict.
In a larger sense, though, it's not just the "ground rules" that are in conflict, but the stories of characters with radically different backgrounds, be it She and Allan, or Frankenstein and the Wolf Man. One might say that what is being "rallied" in such crossover-tales is not any sort of "alliance" between the focal characters, but of the "spirit of monstrosity," just as other ensemble-cast films usually rally spirits of romance, heavy drama, slapstick comedy and so on.
Now, as I pointed out earlier, "hero rallies" and what I have termed "demihero tallies" are fairly common. Since both personas are dominantly positive in tone, it's become common to feature crossovers between such characters. The personas of "monsters" and "villains," however, are meant to be negative in tone: both personas primarily exist to be defeated by the forces of life and/or justice.
That said, it's more typical to see "monsters" as central to particular narratives than it is to see "villains" in the same position. The monster is the dark side of the demihero, even as the villain is the dark side of the hero. The genre of horror is largely about exploring the nature of the monster, while any demiheroes tend to play secondary roles. In contrast, in adventure-fiction and its congeners, the villain exists to define the hero. Monsters are thus often centric, while villains are non-centric.
Often, when monsters or villains are made the stars of continuing features, they are made to battle "the menace of the month," just as heroes do. For instance, the cover of the first issue of DC's 1975 feature THE JOKER shows him visually dominating other Bat-villains, though in the story proper he's only engaged with fighting Two-Face over some slight.
Similarly, in the 1940s FRANKENSTEIN series by Dick Briefer, the Monster's first adversary is his creator Frankenstein, whom I view as more "demihero" than "hero." Doctor Frankenstein does not last long as the Monster's main opponent, but before the scientist vanishes from the series, he creates one or two "monsters of the month" to battle his greatest creation.
Now, the presence of two monsters in the Frankenstein story doesn't really constitute a "monster rally" like that of the 1943 Universal film. In that film, both the Monster and the Wolf Man comprise an ensemble, for they are of equal interest to the ideal viewer of the story. Not so the croc-monster in the FRANKENSTEIN comic; he's simply an opponent for the main character. "Villain rallies" in which both villains have been the stars of their own serials-- even if they did not start out that way-- are much rarer, but Two-Face is not the equal of the Joker in the Joker's book. The earliest example of a "villain rally" wherein both evildoers have been featured characters in their own narratives is this 1964 crossover of Walt Disney properties: the comic villains the Phantom Blot and the Beagle Boys, both of whom had enjoyed their own comic books by the time this issue appeared.
Thus, in the third and last part of this essay-series, I've compiled for my own amusement the main ways in which "monster rallies" usually take place. "Villain rallies," which are less common, are pretty much subsumed by the same narrative rules, and so I won't make separate reference to them.