There's a good chance that Mothra, who died in Godzilla: King of the Monsters, may return in Godzilla vs. Kong. If so, it would be Mothra's second MonsterVerse appearance. Considering the scale of King of the Monsters, it seems unlikely that Warner Bros. and Legendary will follow it up with a movie that only includes two Titans, so more monsters may be confirmed in the near future.
The final battle in Godzilla: King of the Monsters consisted of a brutal two-on-two match, with Godzilla taking on King Ghidorah and Mothra going head-to-head with Rodan. Godzilla was struggling against a powered-up King Ghidorah, while Mothra was able to incapacitate Rodan with her stinger, but unfortunately, Mothra's wings were burned beyond repair by Rodan's fire. Instead of succumbing to her wounds, Mothra took a direct blast from King Ghidorah and allowed her life essence to flow into Godzilla. In a moment that originally belonged to Rodan, Mothra sacrificed herself so that Godzilla would have the power needed to beat Ghidorah.
Of the four Toho kaiju featured in King of the Monsters, only Godzilla and Rodan made it out alive. Both can easily return in Godzilla vs. Kong, and there's evidence that Mothra may be along for the ride as well. First of all, the credits of King of the Monsters sets up Mothra's return by revealing the existence of a giant egg, and the credits tease that the egg may belong to Mothra. If this is true, then the egg could soon hatch and lead to the birth of a new Mothra, as is often done in Toho's movies. Mothra is essentially an eternal creature on some level, constantly being reborn.
The theory that Mothra is secretly in Godzilla vs. Kong is supported by the fact that one of the three returning cast members from Godzilla: King of the Monsters is Zhang Ziyi, who played Dr. Chen, and for one brief scene, Dr. Chen's twin sister. Ziyi's characters are modernized versions of the Twin Fairies from the classic Toho movies. In the original films, the Twin Fairies have a deep connection with Mothra and are heavily involved in Mothra's stories. That connection is translated to the MonsterVerse when Dr. Chen explains her family's history, and when her sister watches Mothra's metamorphosis.
Interestingly, there were originally plans to expand on Dr. Chen's role in a post-credits scene for the film. While Godzilla: King of the Monsters had a credits scene featuring Charles Dance's Jonah and one of Ghidorah's severed heads, the filmmakers planned on filming another one with Dr. Chen and her sister. In that unfilmed scene, the sisters meet inside an ancient temple in Tokyo, and one of the sisters expresses her concerns with something they're about to do. They then enter a chamber in which a second Mothra egg is revealed with another set of identical twins signing the Mothra song, thus trying to hatch the egg (via ComicBookMovie).
Sure, the scene was never filmed, but considering that it was once on the docket, the storyline can be revisited in the future. Zhang Ziyi being in Godzilla vs. Kong means that both sisters can be in the new movie. Their psychic connection to Mothra can be an asset for Monarch (if that's a route the franchise chooses to go in), especially if they need to acquire the Titan's help. Plus, with Godzilla set to battle Kong in the film, the current King of the Monsters can use all the help he can get. So it's possible that Mothra, who is one of Toho's most popular creations, could emerge once again as an ally to Godzilla in Godzilla vs Kong.
