Giant Rock

The Megalith Zygarde is an entity created by Lysandre in the anime, using the Megalith, an ancient, gaint stone that contains massive amounts of Primal energy.

Biology

The Megalith Zygarde has roughly the same body shape as a 50% Zygarde, except it is much larger. it's upper body is primarily made out of stone, with a glowing red crystal serving as its "eye", whereas its lower body is composed out of vines. These vines also serve as a defense mechanism to protect the core body from attacks. The Megalith Zygarde is partly hollow inside, with its "innards" being composed out of glowing red crystals, with Chespie, its power source, at the center. these red crystals will likewise grow and shift to serve as a defense mechanism for its energy source. Even after the power source is removed, Lysandre still managed to power up the Megalith Zygarde remotely using a control device coupled to his gauntlet. Due to it's vast size and mindless nature, the Megalith Zygarde is extremely difficult to slow down, let alone defeat. In order to stop it, Steven, Diantha, Ash, Alain, Malva and Professor Sycamore, as well as Kalos' eight Gym Leaders, needed to have all their strongest Pokémon team up against it. Even then, after Lysandre powered it up again, Squishy and Z2 had to transform into a Complete Zygarde to stop it, and only managed to do so after Ash's Pikachu, Greninja and Alain's Charizard destroyed Lysandre's control device.

Plot

The Megalith first appeared in SS031. Lysandre had his scientists research it, but Mega Rayquaza tried to attack them. Alain and Steven attempted to fend off the Legendary Pokémon, but the Megalith disappeared. During the events of SS032, the Weather Trio battled over the Megalith, while Alain and Steven fought off Groudon and Kyogre long enough for Lysandre to secure the gaint stone.