Goh's first Magikarp, which was regular-sized, was reeled up by Ash during their visit to the Resort Area in the Sinnoh region. After it was reeled in, Goh asked Ash if he could catch it for his Pokédex, which Ash agreed to, and Goh caught it with a Poké Ball.
Goh's second, much larger Magikarp, also known as "The Lord of the Lake", was reeled in by Goh with the help of Ash, Jessie, James, Meowth, and Wobbuffet after being attracted by one of the lures that James had suggested for beginner fisherman. Goh attempted to throw a Poké Ball at it, but Magikarp deflected it. In response, Goh sent out his Oddish and used its Sleep Powder to send it to sleep, before using another Poké Ball on it, capturing it successfully.
Goh later used the large Magikarp to distract members of the Matori Matrix, an elite team made up of Team Rocket's strongest grunts, led by Matori. When Goh had it use it's Splash attack, it's weight easily caused a large Pokémon such as Rhyperior to be thrown into the air like a rag-doll.
When Goh decided he wanted to enter the Magikarp High Jump Tournament, he decided to use his big Magikarp yet again. However, when he sent it out of its Poké Ball, it was revealed that it had become fat due to not exercising. Goh inspired Magikarp to start an exercise training regiment after showing it Kasukarp's Magikarp, which had several abs. Ash and Pikachu also decided to join in, and by the end, Goh's Magikarp had become ripped with muscles, just like Kasuking's. Goh managed to easily reach the finals of the competition, where he was up against Kasuking. When Goh had his Magikarp jump, it jumped so high it breached the atmosphere and reached space. Due to the rules making it so they can only measure the jump when they reach the ground, Goh was disqualified and Kasuking won by default. Goh waited for Magikarp to come back down, but reluctantly gave up when it seemed that it was never coming back.
Later, when Goh and Ash were on Slowpoke Island, they were tossing a Slowking's Shellder back and forth, being controlled by it, when Goh's Magikarp suddenly came back down and crashed onto Slowking's head, putting an end to the controlling. Goh was very happy to see that Magikarp had returned safely, previously assuming that it had been lost in space forever.
- The giant Magikarp is Goh's second giant Pokémon. Golurk was the first.
- It is the second group of duplicates Goh has, the first being his three Dustox.
At the Cerise Laboratory