After being forced to evolve as a part of an experiment by Rocket-dan, this Gyarados went on rampage until Wataru managed to catch it. It was later used to help stop the battle between Kyogre and Groudon.
~!In the new series, it was used to battle Dande's Lizardon in the finals of the Pokémon World Championship, where it was defeated, despite possessing both a type advantage and the ability to Dynamax.!~
(Source: Bulbapedia)
