Slowpoke debuted in Sport of the Hunt, where it was caught by Leaf. It realizes moments later it was caught, and tried to refuse it afterwards. It was later used to open up a waterfall, finding Dratini.
It cameos in Blind Fury, being sent out along with the rest of Leaf's group. It doesn't realize this until minutes after the fact.
In Psychic Readings, Slowpoke is sent out in a battle against Emerald's Frisk. Slowpoke loses, however, it reveals the move Amnesia.