It’s a Tarasque, a southern France based mythological beast that has poisonous breath that shot out of its mouth in thick vapors. It’s described as a Lion headed monster, with six pairs of paws of a Bear, the shell of a Turtle, and ending with the scaly tail of a serpent that ends in a spurred shape of an arrowhead, or the bottom half of a fish, and a mouth full with sword-like teeth as sharp as horns. In French Heraldry, it’s been described as a green (sinople) dragon swallowing a man, with golden scales covering it’s hide. The two ‘imaginary’ Legendaries connect to it as well through the Bonachus and the Leviathan.
I’m pretty sure it’ll be Zygarde-like in looks, but be its own, new Pokémon. Probably look like a ‘Paradox’ of Kalos’ third legend, but obviously be an entirely separate creature co fused for Zygarde.