Pokémon Sword & Shield has scrapped the Mega Evolution and Z-Move mechanics from the earlier games and replaced them with the Dynamax & Gigantamax powers, which transform a Pokémon into a giant monster. The recent unique Gigantamax forms that have been revealed seem to have been inspired by creatures from the kaiju movie genre, including the Godzilla and Gamera franchises.
The giant movie monster genre originated in Japan and the films were mostly made with that audience in mind. The Pokémon franchise has been referencing kaiju movies since before there even was a Pokémon, as the concept artwork for Capsule Monsters shows a monster resembling Godzilla fighting a monster that looks like King Kong.
The introduction of giant Pokémon to Pokémon Sword & Shield has allowed Game Freak to add all of the kaiju references that it has clearly wanted to use for years. The new Gigantamax Pokémon includes several nods to popular movie monsters, including Gamera, Godzilla, and Mothra.
Gigantamax Charizard & Burning Godzilla
Japanese media loves its transformations and even Godzilla has a power-up that he has called upon in certain films. In Godzilla vs. Destoroyah, Godzilla was affected by a volcanic eruption hitting a hidden uranium deposit, which caused burning marks to appear across his body and greatly increased his strength, which was a transformation referred to as Burning Godzilla by the fans. The Burning Godzilla form most recently appeared in Godzilla: King of the Monsters, when Mothra sacrifices her life to give Godzilla her remaining energy, which he uses to power up and destroy King Ghidorah.
Charizard's popularity has ensured that it will keep receiving power-ups and the Gigantamax form is no exception. The fiery body of Gigantamax Charizard and the flame breath of its G-Max Wildfire attack just scream Burning Godzilla, making it an obvious homage to the famous movie monster.
Gigantamax Butterfree & Mothra
Mothra is one of the most prominent kaiju in the Godzilla movie franchise. She originally debuted in her own movie, before crossing over with Godzilla and becoming a popular character in her own right. Mothra has appeared in numerous kaiju films and it's fitting that a Gigantamax Pokémon would exist that is visually similar to her.
It would have made far more sense to give a Gigantamax form to one of the several Pokémon that are already based on a real-life type of moth (such as Venomoth), but Butterfree was one of the most popular Pokémon from Pokémon Red & Blue, so it became the giant bug monster in Pokémon Sword & Shield.
Gigantamax Drednaw & Gamera
Gamera is not part of the Godzilla movie franchise and is seen by some as a rip-off of the concept. Gamera was the star of his own movie franchise and has been referenced numerous times in anime, manga, movies, TV shows, and video games.
Gigantamax Drednaw was one of the first Gigantamax creatures revealed and it should have been obvious from the start that Pokémon was taking inspiration from the kaiju genre. The big turtle monster with its little arms seems like an obvious shoutout to Gamera and similar creatures from the Godzilla movies, such as Anguirus.