A god worshipped by the Seatopians (basically Atlantis), he was sent up to the destroy humanity. While Megalon had some interesting abilities, he acted more like a child when unleashed on the world. As usual, Godzilla was recruited by a robot named Jet Jaguar (don’t ask) to stop Megalon, who eventually retreated back to Seatopia.
Resembling a giant beetle, we really could get an amazing redesign of this monster. With most of the titans we’ve seen resembling larger versions of already existing or extinct life forms, Megalon has no reason not to show up in a future movie. His re-appearance could range from being a deity worshipped by a civilization aeons ago, or as simple as a giant bug Godzilla crushes under his feet. Either way, it would be great to see the giant bug once again.
The first time we were introduced to Battra, a demonic looking bat, he was described as having been created by the Earth itself to defend the planet from an ancient civilization, but Battra became uncontrollable and the earth was forced to create Mothra to put an end to Battra. Aeons later, both Battra and Mothra were re-awoken and another battle ensued, with Godzilla being caught in the middle, until both monsters were forced to put aside their differences to destroy Godzilla, with Battra losing his life in the process.
To me, Battra is a very underrated monster and could make for a very welcome to the Monsterverse. Even the story he was originally given wouldn’t need to be altered at all for a more modern re-telling. Mothra was a very popular character in KotM, and having her dark counterpart show up would be a perfect balance for her. Plus, it would be an interesting way to show how ancient civilization on earth who co-existed with the titans, functioned before their eventual destruction.