It's been 25 years since Pokémon made its debut and fans were introduced to Ash Ketchum. Ash is most known for his Pikachu, the official mascot of Pokémon and The Pokémon Company, with the little yellow mouse being his closest companion and signature Pokémon. Despite how long they've been together and how close Ash and his Pikachu are, it wasn't his electric friend that helped him win his first real battle.
In Episode 3 of Indigo League, "Ash Catches a Pokemon," Ash wanders a forest trying to make his first catch: a Caterpie. He doesn't fight the Caterpie, opting to catch it as is, and is successful. This is where we first learn of Misty's fear of bug-type Pokémon. Ash is ecstatic with this first step to becoming a Pokémon Master and he decides to let Caterpie spend time outside its Pokéball, much to Misty's dismay. Misty ends up insulting the Pokémon, and her yelling about how gross Caterpie is hurts the little bug's feelings. Since she hates his Pokémon so much, Ash decides that they'll go on without her. Unfortunately, there's only one path out of the forest, so Misty has to follow them to get out.
Ash and Misty camp out for the night, and the next morning Misty wakes up with Caterpie sleeping right next to her, freaking her all the way out. Dejected, Caterpie sadly returns to its Pokéball, leading to another lecture from Ash about how Pokemon have feelings too, seemingly convincing Misty to reconsider her approach. A Pidgeotto appears, and Ash wants to catch it. But, instead of using Pikachu, he sends out Caterpie, who is weak against Flying-type Pokemon. Caterpie, of course, gets banged and swapped out for Pikachu. Ash catches the Pidgeotto, but is instantly set upon by Misty about how his ignorance put Caterpie in danger because of the type-disadvantage.
Her lesson is interrupted by Team Rocket, who arrive to steal Ash's Pikachu. They formally challenge him to a battle -- Ash's first ever -- and he struggles with managing two Pokémon at once on the battlefield. Pikachu is blinded by Koffing's sludge attack, and Caterpie is still weak from its fight earlier, so Ash sends his new Flying-type out. Unfortunately, Pidegeotto is quickly knocked out, and Ash has no choice but to send out Caterpie again. Caterpie is on its last legs when Ash remembers its String Shot attack. This is just enough to beat Ekans and Koffing, and even Meowth when he enters the fray. This sends Team Rocket running and nets Caterpie enough experience to evolve into Metapod -- not bad for Ash's first real battle ever.
This just goes to show that even the weakest Pokemon are capable of great things when given the chance. Everyone thought Caterpie was too weak, first for being a little bug-type, and second for having been hurt in its fight against Pidgeotto, but it proved that one shouldn't judge a book by its cover.
Beyond the lesson of the day, though, it's surprising that Ash's first win on his long Pokémon journey wasn't with his constant companion, Pikachu, but instead was with Caterpie, of all Pokémon. It's even more surprising when you factor in that he caught Pidgeotto in the same episode, a Pokémon who'd go on to become an MVP of Indigo League. But it fits for an episode all about moving past assumptions.