Goh first encountered Tentacool in his home town of Vermilion City in the Kanto Region at one of its harbors. A swarm of Tentacool had approached the harbor, and Goh decided to catch one. After singling out the one he thought was the strongest, he attempted to throw a Poké Ball at it. However, the Tentacool he was trying to catch dodged the ball, resulting in it hitting a Tentacool that was swimming behind it, causing it to be caught instead.
Goh later had Tentacool use Ice Beam to freeze some of the flood water caused by Golurk‘s Rain Dance during the food crisis at Cerise Laboratory so that he could reach his Cascoon. Afterwards, it continued to help freeze water to keep it from interrupting Cascoon's evolution process.
At the Cerise Laboratory