Pokémon 11x21

Ash and Co. arrive at a lakeside Pokémon Center. Jessie's Dustox sees a shiny Dustox, and follows it to the lake. Jessie releases her Dustox, allowing it to mate. This is related to the Season 1 episode 21, "Bye Bye Butterfree" where Ash releases his Butterfree so it can mate with a pink Butterfree.