Pidgeotto was Ash's second ever Pokémon captured. It was always used to search for missing Pokémon or people and was used a fair bit in Gym & League battles throughout Kanto.
After Pidgeot evolved, it went into battle against a flock of Spearow & Fearow that were terrorising Pidgey & Pidgeotto in Viridian Forest. After the battle was done, Ash decided to leave it in the forest to lead the flock, but left with the promise that he would someday return for it.
WTF. I NEVER understood why the second he got an amazingly strong Pokemon, he'd leave it and NEVER came back. Also, Pikachu, with all that experience, NEVER won a thing. It was losing to amateurs in every season. - _ -