The novelization thing isn't really a leak, it's an official preview which you can exploit to find mentions of keywords anywhere in the novel.
Spoilers from the novelization follow. I'm not sure if all of these apply to the film as well, but based on what I know at least some do.
MUTO Prime's carcass appears being dissected along with both the carcasses of the male and female MUTOs. Both MUTOs are shown repeatedly through stock footage, and a second, living male MUTO is one of the Titans awakened by Ghidorah and is present for the film's final scene.
Shinomura is mentioned by name in a recap of Monarch's origins and Godzilla's discovery.
As mentioned already, Methuselah is the mountain Titan from Germany. He's described as a horned monster with a mountain on his back. Scylla is a tentacled spider-like monster from Arizona, and is specified as female. Behemoth appears from Brazil, and its most notable feature are its colossal tusks.
Other Titans include the Leviathan from Loch Ness, the Kraken, and Mokele-Mbembe which appears in Sudan. The carcass of a Titan called Margygr, named after a mermaid from Greenland's mythology, appears as a carcass. It is described as a reptilian humanoid monster with nostrils above its eyes.
Kong, full designation Titanus Kong, is mentioned and shown on video footage several times. There is a scene later in the story where he is actually described while hanging out on Skull Island.
Leafwings escape containment on Skull Island and show up at the very end of the story.
Again, not sure if all of these makes it into the film as well, but this is all definitely in the novelization. Novelizations are known to have some elements and scenes not present in the film itself, and this was the case with G14.
