Every Pokémon's design is somewhat inspired by something from reality. Sometimes it is a really simple influence like how Voltorb is just a Pokémon based on a Pokéball, but more often than not there are plenty of creature designs that have multiple layers.
One of my favourites is how Decidueye is based off an extinct Hawaiian owl with long legs, as well as deciduous trees which shed their leaves and "die" in winter.
Gigantamax Pokémon were one of the newer gimmicks introduced in Pokémon Sword and Shield. They're similar to Dynamax forms, only the Pokémon that were capable of this new temporary evolution have dramatically altered designs.
Not every Pokémon got one and the bulk of those that did were new creatures. Some old favourites did get new forms too. The one thing they have in common is that each Gigantamax Pokémon form expands upon the inspiration behind their original design.
I've gone through the entire list of available Pokémon that can Gigantamax (as of the Isle of Armor DLC) to find the most interesting designs with fascinating inspirations.