Guzzlord is Zygarde from a universe where the use of some kind of nuclear energy (sea mauville, alola thermoplant) has gone completely out of control, destroying most of the planet's lifeform. In an attempt to save everyone Zygarde as the guardian of the environment tried to absorb the nuclear energy and got corrupted. Being the only thing that survived the chaos without having to leave the planet behind like humans did, but now he is also a destruction power himself.