The Pinkan Berry is a berry that grows on Pinkan Trees, which are found only on Pinkan Island of the Orange Islands. As noted in their debut (and only) episode, if a Pokémon eats the berries, they end up turning pink (either partially or fully).
Ash's Pikachu ate some of the berries on the island when he, Misty and Tracey landed there by accident after getting washed up on the beach by a series of whirlpools -- Ash was initially worried that Pikachu would stay pink forever, but Officer Jenny (who ran Pinkan Island's Pokémon reserve) assured him that it would only be temporary for his Pikachu. She elaborates by explaining that, since Ash's Pikachu doesn't live on the island like the wild Pokémon that live on the reserve and had only eaten the berries once, he'll only stay pink temporarily -- Pokémon only stay pink permanently if they eat the berries as a primary food source.
Ash was relieved by this, jokingly noting that he wouldn't have to call his partner Pokémon a "Pinkachu." Ash's friend, Tracey, deduced that there must be some kind of natural chemical in the berries that causes Pokémon to turn pink after eating them.
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Ash's Pikachu eating a Pinkan Berry, thus becoming a Pinkachu.
.jpg "Pokémon eating Pinkan Berries (2).jpg (30 KB)")