In what promises to be the biggest cinematic throwdown since Batman v Superman, Godzilla and King Kong are finally rekindling their age-old rivalry in the upcoming Godzilla vs. Kong. When a gigantic lizard monster and an oversized monkey get into an old-fashioned donnybrook, there can be only one victor.Or can there? If we've learned anything from Batman v Superman and the many other hero vs. hero crossovers in pop culture, it's that these stories usually culminate with the two combatants putting aside their differences to battle a common foe. In this case, will Godzilla and King Kong team up to take on Mechagodzilla or Mecha-King Ghidorah? That's a popular theory among fans, and there's plenty of evidence to support it, including a tantalizing couple of moments in the trailer for Godzilla vs. Kong. Here's why Godzilla and King Kong's big brawl may be just an appetizer for the true fight to come.Could this red-eyed monstrosity in the Godzilla vs. Kong trailer actually be Mechagodzilla?Legendary's Godzilla MonsterVerse: The Story So FarGodzilla vs. Kong is actually the fourth film in a shared cinematic universe that began with 2014's Godzilla reboot. Dubbed "The MonsterVerse," this series also includes 2017's Kong: Skull Island and 2019's Godzilla: King of the Monsters. In this universe, humanity is under constant threat from gigantic monsters called Titans or MUTOs (Massive Unidentified Terrestrial Organisms), who are attracted to humanity's nuclear technology. Godzilla himself is basically nature's immune system. He wakes up every so often to fight off these Titans and prevent the world from being completely overrun.While there are a handful of characters who appear in multiple movies (Millie Bobby Brown's Madison Russell, Kyle Chandler's Mark Russell, Ken Watanabes Dr. Ishir Serizawa), it's really the scientific organization Monarch which ties the larger universe together. Monarch is formed after World War II to study the Titans and attempt to hide their existence from the general public. It's this group that sets the events of Kong: Skull Island in motion, and they're the ones tasked with dealing with a new wave of Titans in Godzilla and Godzilla: King of the Monsters. The organization will continue to play a major role in Godzilla vs. Kong and, in fact, they may be the ones responsible for setting the final battle in motion.The post-credits scene in King of the Monsters certainly hints at this outcome. That movie ends with a glimpse of Charles Dance's character, ecoterrorist Alan Jonah, approaching the slain corpse of King Ghidorah, heavily implying his group has plans to study and even weaponize the remains of Godzilla's powerful nemesis. That certainly seems like a seed that'll bear fruit in Godzilla vs. Kong.Who Are Mechagodzilla and Mecha-King Ghidorah?The Godzilla franchise has seen many incarnations over the years, but one thing is constant. Inevitably, the king of the monsters starts battling giant robots. Godzilla's feud with Mechagodzilla dates all the way back to 1974's Godzilla vs. Mechagodzilla. In that movie, Mechagodzilla is portrayed as an alien weapon constructed of "space titanium" (which is clearly far superior to regular titanium) sent to conquer Earth for its creators. However, in most of its later appearances, Mechagodzilla is instead depicted as a terrestrial robot charged with protecting humanity from Godzilla. Ironic, considering Godzilla is just trying to save us from ourselves.Whatever its current origin story, with its vast armory and armored shell, Mechagodzilla usually puts up a pretty good fight. But in the end, it's only ever a matter of time before Mechagodzilla winds up decapitated and explodes.Similarly, Mecha-King Ghidorah has a long history duking it out with Godzilla, starting with 1991's Godzilla vs. King Ghidorah. In that movie, humans from the future use their technology to revive King Ghidorah and give him a technological upgrade. For once, Godzilla's nemesis is shown fighting in defense of humanity rather than trying to destroy it.Godzilla vs. Mechagodzilla vs. Kong?As much as we're all pumped to see Godzilla and King Kong pummel each other, we highly doubt the new movie will show them at each other's throats the entire time. Both characters are really the heroes, at the end of the day, even if Kong's story tends to end in tragedy. Kong only cares about protecting whatever comely human lass has caught his eye lately, while Godzilla is basically the MonsterVerse's version of Galactus - a great destroyer who maintains balance in the universe.Eventually, these two monsters will realize they shouldn't be fighting each other. That's probably where the movie's true villain comes in. As we covered already, the post-credits teaser in King of the Monsters hints that Monarch is using the remains of King Ghidorah to build a terrible new weapon. And we might even see a glimpse of that weapon in the first trailer for Godzilla vs. Kong. If you pause at the three- to four-second mark, those glowing red eyes sure seem like theyd belong to a mecha-giant monster of some kind, dont they?The old-school Mechagodzilla.The trailer also shows a computer screen that seems to display the schematics for a mechanized Titan. Heck, it could even be that Mechagodzilla is running around wrecking cities in a Godzilla skin suit, and thats why everyone is so mad at Godzilla!Or maybe it's just that, having endured several giant monster rampages in the last few years, Monarch's scientists are obsessed with building a weapon that can stop Godzilla and all other Titans in their tracks. Using a combination of King Ghidorah's carcass and the finest tech 2021 has to offer, Monarch will unleash a greater threat than anything it's been trying to stop. Whether that threat turns out to be Mechagodzilla or Mecha-King Ghidorah (or both), the end result is the same.It's an understandable, if misguided, mission. Nobody wants to live n constant fear of an enormous reptile stomping through their apartment complex. But as is so often the case in science fiction, mankind may be guilty of underestimating its own creation. We know from King of the Monsters that Ghidorah is an ancient alien trying to terraform the planet. What if Mecha-King Ghidorah retains its memories, causing what should be a defensive measure for humanity to become a greater threat than Godzilla or King Kong could ever be? Godzilla and King's wrestling match could be the least of our problems.And then there's also these Funko POP figures for the film, which feature variations on Godzilla and Kong... including a Kong with RED eyes. Maybe he just needs some visine... or could we be looking at a Mecha Kong as well here? There is a precedent for that, and he's called... Mechani-Kong!Godzilla vs. Kong will debut in theaters and on HBO Max on March 31. For more on this epic crossover, find out why Kong has gotten so big and cast your vote to decide who wins in this titanic monster battle.Jesse is a mild-mannered staff writer for IGN. Allow him to lend a machete to your intellectual thicket by following @jschedeen on Twitter.