Just realized: the giga in the dossier IS Demon King, Mei Yin's giga. As far as I know, he was the only one of his kind during the events of the Island :
Pétard Giga Demon x 4 Paquets - Pétards - Milleproduits
True Ultimate Battle Skill, GIGA Demon Slay
Selection de Petards Demon Giga Cool (24 Gros Petards) - Sparklers Club
My 41-45-Headed Giga Demon Barney By Brad Bartley, 46-50-Headed Giga Demon Barney coming soon!! | Fandom