It was caught as a Pidgeotto in the Viridian Forest. Then when he was heading to Orange Island to bring the GS Ball back to Professor Oak as requested on the way, there was a bunch of Pidgey and Pidgeotto that did not get along with some Spearows with a Fearow leading them. It appeared that the Fearow was the very same Spearow that Ash threw a stone at the start of his journey. Then Ash asked his Pidgeotto to convince the Pidgey and Pidgeotto that he wants to help them. When Ash's Pidgeotto was fighting against the Fearow and Spearow, Pidgeotto was hurt and then he suddenly evolved into a Pidgeot. When the Fearow was weak, Ash tried to catch it, but failed to do so. So with the help of Ash's Pikachu, they were able to chase the Fearow away. Then Ash released his Pidgeot to protect the Pidgeotto and Pidgey promising to come back for it after he delivered the GS Ball, a promise he hasn't yet fulfilled.