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.

Spoiler

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)