Zacian and Zamazenta represent the irresistible force paradox (also known as the spear and shield paradox): Zacian represents the unstoppable force, the sword that can pierce and cut through anything. Zamazenta represents the immovable object, the shield that cannot be pierced through or broken, and is impenetrable. When these two forces meet in battle how can one prevail over the other? The "paradox" is flawed because if there exists an unstoppable force, it follows logically that there cannot be any such thing as an immovable object and vice versa.
The Pokedex has a staggering 400 Pokemon to find and catch, the most in any Pokemon game to date.