Hydreigon is the fourth Pokémon in Cameron's team, and was also his "secret weapon". Cameron sent Hydreigon as his first Pokémon, during his battle against Ash.
It first battled Ash's Boldore. Both Pokémon fought hard but Hydreigon easily dodged all of Boldore's attacks, while landing hits with Tri Attack and Dragon Rush, and finally knocked him out with Dragon Pulse, which easily overpowered Boldore's Flash Cannon. Despite the notable size difference, Ash's Oshawott proved to be a reasonable opponent, right away scoring a hit with Aqua Jet. Hydreigon came back and avoided Oshawott's Razor Shell before striking back with Dragon Pulse. Despite taking damage from the hit, Oshawott went on with another Razor Shell, which hit one of Hydreigon's heads, and then scored a direct hit on the Hydreigon with Hydro Pump. Hydreigon fired a Tri Attack, which Oshawott was unable to block, taking more damage. Oshawott fought back with Tackle, but his following Razor Shell was countered by Hydreigon's Double Hit, which was then followed by a Dragon Rush, finally defeating Oshawott. Ash's Pignite, was what finally defeated the beast, though it was no easy battle. Hydreigon scored several powerful blows, but Pignite uses Fire Pledge to force Hydreigon into flight, and then scored a direct hit with a super effective Brick Break, which knocked Hydreigon out.
In Cameron's battle against Virgil, the scoreboard showed Hydregion had lost to one of Virgil's Pokémon.