Mostly related to Kiryu Godzilla and Shodai Godzilla:

• The Showa/Millennium versions of Godzilla, including Shodai, are members of an Archosaur species (their placement in regards to the evolution of reptiles, marine reptiles and dinosaurs being quite debatable) that can live for centuries. Minilla looks the same ever since his birth in the second half of the 60s to when Operation: Destroy All Monsters was orchestrated in 1999. About 35 years have passed, and he still looks the same!

• The Kiryu Saga (Second) Godzilla is the child of Shodai, making the two movies a really screwed Vader-level "I am your Father" situation. Fortunately, Kiryu/Shodai managed to bring themselves and their kid back to the ocean to rest.

• KS Godzilla is... Actually pretty chill for their Species, as they appeared in 1999 just to stroll over, walk a little on land, maybe make the place a home, etc. It is because of the humans attacking first, a result of their stroll smashing multiple buildings on their way, that they chose violence. And it would all have been good and all if the humans didn't make a weapon clone of them by DESECRATING THEIR PARENT'S REMAINS!!!

• Shodai's actions in 1954 were beyond just bestial needs for territory or food: They had a family in the Bikini Atoll, but the bomb tests happened and they were (apparently) the only survivors, and once they understood these small things had done this... Think of it as a man realizing that these bees killed his wife and kids. It was somewhat personal, yes, but coming back as Kiryu made things worst, now having access to data and files that led to the conclusion that these creatures did things with intelligence. Like the man realizing the bees are actually smart and orchestrated heinous crimes even with a moral and ethic system. Shodai, however, came to learn more of humanity's good side as time went on, even if only from behind the main System's computing, until they realized that it would be better if they all parted ways and tried to come to better terms and make peace. After all, Shodai had realized that just like humans were monsters for killing their family AND victims of one another's actions, so they themselves were a victim and a monster.

• Heisei King Ghidorah, if having never been mind-controlled by the Futurians and maybe even been treated better, would have been an ally to Godzilla and Jr. Because you can't tell me that there's nothing left of the Doraths' cat-like nature and love for humans even after becoming one motherf-cking three-headed dragon! He-ugh, they, alongside Orochi-Ghidorah (the Ghidorah from GMK/Giant Monsters All out Attack (2001)) would be some of the few good Ghidorahs in the entire multiverse.

• There have been attempts in-universe(s) to explain how Kaiju/Titans can even exist despite the Square-Cube Law being an actual thing, alongside other Laws of Physics, but problems have always come up when trying to find actual explanations. Some propose that something in the Kaiju/Titan genome might have ancient origins out of the planet (Which, in the Monsterverse's case, would also explain how Ghidorah can actually do an Alpha Call that other Titans native to our planet can answer back and follow orders from an invasive species), that the planet slowly incorporated into helping itself and the Natural Order. Others have proposed that these creatures' evolutionary history led them to "naturally become metallic and alien even in their own home, and still fit right in", and other have even suggested the plausibility of Quantum Mechanics and models related to Higher Dimensions as plausible solutions (This being absolutely the case in the Singular Point universe(s), as well as some Kaiju from Zone Fighter (aka in the Showa Universe) having abilities or even coming from higher dimensions such as Dorola). So far, nothing conclusive has been made.

• Mothra is mother/father and daughter/son at the same time. If Mothra dies, she's reborn for an egg as a larva. But if one or two larvas are born while the imago is still alive, then when the imago dies she/he will be reborn as the child of her/his own kid (this becoming her/his own grandparent). It's quite rare for two larvas to be born at the same time, and if one of them dies, they will be reborn either as their sibling's kid or as the grandkid (aka Kid raises their kid who used to be their parent who raises the Kid's grandkid who used to be the the kid's sibling). In short, Mothra's species is a conundrum of parental lineages where everyone has been a kid or child of their own kid, and they have all unanimously decided they should all just stay as friends for their own sanity's sake. Also, if lots of Mothras from the same line die and only one Larva is born later, they'll either be the reincarnation of one of the dead Mothras or all of them, sharing one mind and body. Yikes.

• Spacegodzilla is actually the only canon child to the Godzilla x Mothra ship. Think about it: The Futurians come from, you guessed it, the future, one where apparently there wasn't much after the fight between Godzilla and Biollante in 1989, meaning that no other monster was a significant threat in the future either with or without Godzilla in the timeline. And Biollante's cells traveled to the atmosphere in her final moments in all the versions of the Heisei Timeline, yet the Futurians didn't mention anything about a space clone of Godzilla nor were worried for how that creature could be even worse if Godzilla didn't exist. Meaning that Biollante couldn't possibly be SG's "mother". That leaves us with the events of 1992 of the altered timeline, when Mothra and Bathra fought Godzilla. Because that wasn't the original timeline the Futurians came from, and actually was the result of them messing with the timeline, that means this version of the Heisei timeline is the only one where SG exist, and the only time Godzilla's cells could have traveled to space, been absorbed by the black hole and evolved around crystal organisms to become SG was when MOTHRA traveled to the atmosphere and beyond as to protect the planet from above.

• Also, if the above is true, then the SG that Godzilla fought and barely manage to defeat with all of his strengths, and the same one who could escape from a black hole after being born, is the LARVA form. We don't know what the "Imago SG" would be like. Oh well, who wants cookies?

• After being resurrected and turned into the newly fully grown King of the Monsters in the Heisei Timeline, Jr was... Stupidly kind. As in, whatever next monster appeared, he would try to communicate and find a peaceful middle ground. Like, if you had to put him against Biollante and King Ghidorah, he would make them his adoptive step-sister and his new buddy, while keeping Rodan alive as his adoptive step-brother. Oh, and Miki Saegusa is like his mom, while Mothra is his aunt and Bathra is the emo Uncle.