If you read ahead, you are assumed to have already watched the movie. Beware of unmarked spoilers.
In Monarch's timeline on Twitter, Ghidorah is dubbed Monster Zero. He's called this in the film, since he's so old and so dreaded there's no other information about him to be had.
One of the film's trailers featured LL Cool J's hit single "Mama Said Knock You Out." The 1996 MTV Movie Awards used the song during a presentation honoring Godzilla as that year's recipient of the Lifetime Achievement Award.
In the anti-Titan protest during the opening scenes, one of the protesters' signs reads Destroy All Monsters.
Believe it or not, "Clair de Lune" can be heard in the background in Terror of Mechagodzilla during a café scene involving Ichinose and Katsura.
One of the Titans, Leviathan, is in Loch Ness, implying that it's the origin of the Loch Ness Monster legend. Toho actually nearly made a kaiju film staring Nessie in 1978 in co-production with Hammer Horror and got far enough into production for posters to be made. Nessie also appears in Godzilla: The Series.
One of the unseen Titans is the Yamata no Orochi, which, aside from being a multi-headed mythological dragon, is indeed a real Toho kaiju (which has only faced Godzilla in manga/comics), partly the inspiration for King Ghidorah, and in at least one continuity, the same species as King Ghidorah.
The primary movie poster (seen on the main page) has Godzilla tangled up in Ghidorah's necks as they fight in a manner highly reminiscent of the iconic poster for Godzilla vs. King Ghidorah.
Several times throughout the film, Godzilla lets out his roar from the 1960s and 1970s, as opposed to his modernized roar.
Mothra's roar is modified from her usual roar in the Toho films.
Just before Ghidorah lands in Fenway Park, he lets out the infamous trill his Showa counterpart uses while flying. A few moments after he and Godzilla lock up in the ensuing battle in Fenway Park, Ghidorah briefly lets out part of his Heisei counterpart's roar.
When Rodan briefly challenges Godzilla at the end of the film, he lets out a modified version of his classic Toho screech.
The Monarch outpost where Mothra's egg was kept is Outpost 61, a reference to the year of her debut, the 1961 film Mothra.
Likewise, Rodan was supervised from Outpost 56. He debuted in the 1956 film Rodan.
Outpost 54, aka Castle Bravo, is the Monarch HQ and the place where we get our first look at Godzilla himself (not counting the prologue). Godzilla debuted in the 1954 film Godzilla.
Kong is monitored from Outpost 33. He debuted in the 1933 film King Kong.
Ghidorah was sealed in the ice at Outpost 32. However, he debuted in the 1964 film Ghidorah, the Three-Headed Monster. That being said, it actually has two references: 32 is half of 64, and the titular alien in the horror film The Thing (another story about a world-threatening monster frozen in the ice of Mysterious Antarctica) terrorized a facility called Outpost 31.
The opening credits are literally a sound-for-sound remake of the opening credits for the original Godzilla film.
Dr. Emma Russell discovered a way to communicate with the Titans, which may be a reference to Miki Sagusa and the Psychic program in the Heisei series. She also mentioned that said communication involves a bioacoustic process, much like how the Hanna-Barbera cartoon had the crew of the Calico summon Godzilla using a sonar device.
The concept of sound being used to Mind Control the kaiju to the human-like and/or extraterrestrial antagonists' desires harkens back to Invasion of Astro-Monster, as does the idea that such sound can in turn be used by the heroes to disrupt the control.
The idea of having the human villains of the movie be terrorists isn’t too far off from the Red Bamboo in Ebirah, Horror of the Deep, only here they’re shown to be eco-terrorists instead of being vague about their villainy.
During a slideshow montage of all the numerous Titans that Monarch knows about, the scientific name shown for Kong is Megaprimatus Kong, which was his scientific name in supplementary material of King Kong (2005).
Monarch's official scientific designation for Godzilla is Titanus Gojira. Gojira is the official Japanese name for Godzilla.
Mothra's scientific designation is Titanus Mosura, with Mosura being the official Japanese name for Mothra.
The Osprey which arrives at the Castle Bravo base is called "Raptor-One," which was the name of a gigantic modified Osprey in the Random House novel Godzilla 2000, and filled a similar role that the Argo does in this film.
In the novelization, Mothra is stated to be worshipped under the name "Mosura" on an Indonesian island, confirmed to be Infant Island in a Freeze-Frame Bonus of the movie, and that some myths about her depict similar but destructive moth deities, a reference to Battra. Dr. Ling Chen and her twin sister are a reference to the Shobijin — being directly descended from a lineage of Mothra's priestesses, all of whom are twin sisters, and being revealed in the novelization to share an almost mystical connection to the moth goddess.
Ghidorah is found under Antarctica, similar to how his Millennium incarnation is found in an ice cave.
Misanthropic humans (or at least humanoids) seeking to use the monsters, chiefly including chiefly King Ghidorah, to terraform the Earth to their liking connects with the Xiliens. However, in this film's case, Ghidorah having its own agenda and the misanthropes in question being unable to control it seems to be a Take That! at the previous Ghidorah incarnations' Demotion to Dragon and Villain Decay.
The concept of eco-terrorists unleashing King Ghidorah (and the main female member of the team having a change of heart) calls back to Godzilla vs. King Ghidorah, though with a more faithful-to-the-original version of Ghidorah (and without the time travel).
In a Freeze-Frame Bonus, you can see that Castle Bravo's anti-Titan weapons are Masers, the go-to option in the Toho films. Colonel Foster's profile on the official Monarch site also name-drops them as one of her expertise areas.
Godzilla’s plates make a pounding sound when they light up, specifically a sound originally used for the final, weakening heartbeats of King Kong in King Kong (1976).
The Multiple Head Case characterization of Ghidorah in this movie (the first time the character has been characterized in such a way) seems to be a more subtle reference to Godzilla vs. King Ghidorah, where the three-headed dragon was created by mutating three single-headed creatures into an amalgamation.
The idea of a scientist teaming up with an evil force out of vengeance against the world isn’t too far off from Terror of Mechagodzilla, in which Dr. Mafune teams up with a powerful menace to destroy the world. The only difference is that it’s eco-terrorists instead of aliens and the Mafune character in question, Emma Russell, makes a Heroic Sacrifice not too different from Mafune’s daughter, Katsura, as a way to help Godzilla defeat the monster. Bonus points to the fact that it was a life changing tragedy that lead to their team ups.note Mafune because of the death of his wife and daughter (the latter of whom was rebuilt as a robot), and Emma with the death of her son Andrew and the implied broken relationship between herself, her daughter Maddie and ex-husband Mark.
A misanthropic researcher unleashing kaiju in revenge against humanity's destruction of the Earth mirrors Goro Maki's deliberate awakening of Godzilla in Shin Godzilla.
Rodan is found sleeping in a volcano. In Rodan, the two Rodans seemingly perish on the side of an erupting volcano, and one is revealed to have survived in Ghidorah, the Three-Headed Monster.
The OxygenDestroyer returns as a military-developed weapon designed to kill Godzilla. However, it's delivered by missile here, while a schematic shows that, within the missile, the Oxygen Destroyer consists of that same capsule with a central-sphere design that it sported in its original incarnation. It also fails to kill Godzilla this time around.
Godzilla and Ghidorah having an underwater battle has happened before. Except in GMK, Ghidorah is the good underdog and Godzilla the unstoppable tyrant. Despite Ghidorah being a flying creature, the GMK version has an affinity to water and between that and having an upgrade from Mothra, he's able to fight Godzilla to a standstill. Until human interference and Godzilla's quick thinking turns the tide in his favor again.
Speaking of that film, another Mythology Gag is that King of the Monsters also ends with the kaiju Big Bad seemingly vaporized at the end, only for The End... Or Is It? to ensue when it's revealed one piece of them is leftover at the bottom of a body of water, with the implication that the Big Bad who has a previously-established Healing Factor can probably regrow from just that in a sequel.
When Ghidorah regenerates his left head after it got ripped off, as it grows back there's a few seconds where his lower jaw is split before it fuses. In some concept art of Ghidorah for Rebirth of Mothra 3, he was portrayed with a split jaw.
After usurping Godzilla's position as Alpha, Ghidorah perches himself on a volcano while regenerating his lost head and calling to the other Titans, with a cross present in the wide-shot. This bears a resemblance to a shot from Ghidorah, the Three-Headed Monster (which marked Ghidorah's first appearance) where he's rampaging across Japan, and is imposed over a Torii gate. It accomplishes the same thing as well, namely showing his alien, borderline demonic nature.
The Titans Scylla and Methuselah are based on Toho kaiju Kumonga and Anguirus respectively. In the case of Scylla this is given an even bigger nod by having her emerge in Arizona, which is where Kumonga appeared in Godzilla: Final Wars.
The concept of Ghidorah Hostile Terraforming the Earth in his own image while clearing humanity off the planet seems like a reference to what Godzilla Earth has done to the world in the backstory of the AniGoji trilogy. Ironically, Godzilla Earth is made out by the trilogy to be Gaia's Vengeance acting to punish humanity, whereas this film explicitly portrays Ghidorah as anythingbutan agent of nature.
An ancient underwater human empire worshipping kaiju as gods is similar to Mu and Seatopia (though those still had humans living there while underwater).
In the underwater city you can briefly see the skeleton of another Titan. These bones have been confirmed to be those of Anguirus, or at least one of his species.
In the novelization, the Sudan-based military forces try to halt Mokele-Mbembe by luring him into a makeshift electric fence made of high-tension wires, without much success. This goes all the way back to the Godzilla franchise's beginning. For bonus points, a similar move was also attempted against Hokmuto in the prior movie, this time involving the wires being stretched over the Janjira containment facility as a net, but with the exact same results.
A nuclear sub is sent to the bottom of the ocean to nuke Godzilla to empower him to defeat King Ghidorah. Of course, this time, it goes far better for the sub and the humans.
In an odd sense, there's one to the 1998 movie when Serizawa walks to Godzilla to touch his snout as the Kaiju King is lying down, similar to what Nick did to Zilla when he first appears.
In the finale, Ghidorah targets Madison after working out she's connected to the Orca device. This isn't the first time Ghidorah was a bit of a child hater. In Rebirth of Mothra 3, he specifically kidnapped many children with the intention of eventually eating them.
After being supercharged by Serizawa's nuke, Godzilla starts undergoing a Phlebotinum Overdose with the main characters scrambling to evacuate before he self-destructs.
Godzilla's Super Mode is him covered in flames and glowing red lines in his skin whilst radiating super-intense heat, akin to his Burning Godzilla meltdown form. Said Super Mode is also referred to as Godzilla entering "critical mass," a nod to the game Godzilla: Unleashed where monsters could undergo a similar transformation of the same name.
Like in the movie's climax, Burning Godzilla's sheer destructive power is ultimately key to killing the movie's Big Bad, and said big bad's wings are physically shot and disintegrated by the heroes to stop him from escaping via flying away during the meltdown.
During the final fight, King Ghidorah strangles Godzilla with one of its heads, causing Godzilla to foam at the mouth. This happened as well in Godzilla vs. King Ghidorah during their first encounter.
When all seems lost for Godzilla in his final battle against King Ghidorah, Mothra sacrifices herself so her atomized remains and life energy are absorbed into Godzilla's body, which revives him, grants him his "Burning" Super Mode, and provides him with the brand new Nuclear Pulse ability. In Godzilla vs. Mechagodzilla II, when Mechagodzilla has destroyed Godzilla's hind brain and paralyzed him, a fatally-injured Rodan sacrifices himself so Godzilla absorbs his lifeforce, which regenerates him and provides the brand new, red-colored, Spiral Heat Ray.
During the final fight, King Ghidorah strangles Godzilla with one of its heads, causing Godzilla to foam at the mouth. This happened as well in Godzilla vs. King Ghidorah during their first encounter.
In the climax, Godzilla gets Mothra's blue-colored life force as a boost to finally take down Ghidorah. He's gotten a power boost in the form of blue energy before back from Ozaki to take down Keizer Ghidorah in Godzilla: Final Wars.
If one looks very closely after Burning Godzilla triple-nukes Boston, a ship called the Calico can be seen among the ruins. The Calico was a major vehicle-location in The Godzilla Power Hour.
Steven Martin is credited as the author of an article about a second Mothra egg discovered by Monarch.
The film's Stinger isn't the first time a Ghidorah incarnation's remains have been dredged up post-defeat from the sea floor by humans for further use, with the camera placing emphasis on the space hydra's head.
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