For as long as the Pokémon anime has been around, it's more or less followed the lead of the games. But one of the series' most touching moments from the early seasons also carries the distinction of being a first for the franchise as a whole. Tha moment, from the Indigo League series, takes place during an episode in which Ash parts ways with his loyal Butterfree.
This scene is actually more important than most fans realize. A uniquely colored Butterfree featured in the episode is essentially an anime-exclusive Shiny Pokémon. Here's the story behind the anime's special Butterfree, and how it predicted one of the games' most interesting additions.
In "Bye Bye Butterfree," the 21st episode in the show's original season, Ash and his group of friends encounter a group of wild Butterfree on their way to Saffron City. The Butterfree are in the midst of mating season, and are seeking partners to lay eggs with. One bug that stands out among the group is a pink female Butterfree. Ash's trusty Butterfree tries to attract her, but she remains aloof, in spite of his constant attempts to impress her.
When Team Rocket tries to capture all of the Butterfree, Ash's Butterfree comes to the rescue -- along with Pikachu -- to save his comrades. After seeing him in action, the pink Butterfree finally falls in love with Ash's Butterfree, making Ash realize that it's time to release his Pokémon -- the first time someone had ever done so in the anime. Ash reminisces about Butterfree, looking back at catching him as a Caterpie, but as heartwarming as this sequence all is, it's actually Butterfree's pink companion that's important here.
The female Butterfree in the episode has a pink body instead of the dark purple the species usually has. This makes her the first alternately colored Pokémon in the anime, appearing long before the games introduced a similar concept in shiny Pokémon. Pokémon Gold and Silver, officially introduced Shiny Pokémon, Pokémon with exceptionally rare colorations and a special shimmer whenever they are fought in the wild. Of the many Pokémon to get a Shiny form was Butterfree, who got pink wings and limbs. There's even dialogue in the game referring to a pink Butterfree, possibly referencing the anime. This coloration is inverted from the pink Butterfree in the anime, however, making her incredibly unique, even today.
Since the original episode, that same pink Butterfree has reappeared via flashbacks. Her love story with Ash's Butterfree was also retold in Pokemon The Movie: I Choose You!, albeit in a different way. This peculiarity and ability to stand out in the franchise for over two decades show why Ash's Butterfree was so taken with her in the first place.