Misty's Gyarados is a Pokémon that Misty caught in Kanto. It still resides with Misty in her Gym. It is one of Misty's most powerful Pokémon.
When Misty first returned to Cerulean City, Gyarados was attacking the trainers who were looking after the Gym and it was in such a rage it couldn't return to its Poké Ball. It was locked in a cage to protect it and others around it. When the Pokémon Brothers arrived they challenged Misty for ownership of the gym and during the battle they ganged up on Misty's Corsola and Gyarados looked concerned for it. Then the Tentacruel attacked Gyarados and Misty protected it with her body and nearly drowned. This just made Gyarados angrier and he broke the cage, saving Misty in the process. Gyarados then defeated the Pokémon Brothers with a Hyper Beam and a Hydro Pump combination.
(Information as to specifcally when or HOW Misty caught a Magikarp or Gyarados is needed)