Well fierce's theme should be inspired by One winged angel theme with a hardcore mix of Godzilla Theme (2014), and Unfolded revenge from mother 3. I would be epic and awesome.

Ok Fierce The Bandit and Fierce's origin has the combined roars of Legendary Godzilla, Legendary King Ghidorah, and Fake Groundon but sounds like palkia from Pokemon films and Super smash bros brawl.

https://www.youtube.com/watch?v=3FKdjVre078

Space Fierce The Bandit has a combined roars of Space Godzilla, Orga (JP), and King Ghidorah.

Dark Fierce has a combined roars of Giratina, destroyah, and Chandelure.