What was the reason why Anguirus, Kumonga, or Baragon. However, none of these theories panned out.
In Godzilla: King of the Monsters, it was revealed that the only Toho kaiju being added to Legendary's MonsterVerse were the third M.U.T.O. - were all original to the MonsterVerse, with each of them but the third M.U.T.O being linked to mythological figures. It was also discovered that 11 more unseen Titans exist somewhere in the MonsterVerse, but like the four that appear in King of the Monsters, none of them are connected to Toho's properties.
Why did King of the Monsters create its own Titans, instead of drawing upon Toho's library of kaiju? Godzilla: King of the Monsters director Michael Dougherty has explained in the past that using Toho monsters is expensive, saying that the rights to each kaiju comes with a "price tag" [via Biollante, Anguirus, and Gigan as monsters he would liked to have used. But acquiring the rights to them or any other Toho monster would have been a problem for their budget. Due to this issue, it was decided that instead of just having a movie with Godzilla, Rodan, Ghidorah, and Mothra, they could still use more monsters, but in a different way. For that to work, brand-new Titans would have to be designed.
That's how the MonsterVerse ended up coming with Behemoth, Scylla, Methuselah, and the third M.U.T.O. Of course, their inclusion over fan-favorites such as Anguirus may have been disappointing at first, but it's understandable why this had to happen. In fact, their absence could be for the best. It's worth wondering what the reaction would have been if major Toho properties appeared in such small roles. It would make more sense for creatures like Gigan to be saved for when they can be more integral to the plot, not as random side characters in an otherwise packed film.
Currently, Monarch explores the Hollow Earth in Godzilla vs. Kong.
More: Godzilla: Every New King Of The Monsters Reveal From The Director
