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Ash's Fearow (Japanese: サトシのオニドリル Satoshi's Onidrill) is a Pokémon which Ash caught in the Electric Tale of Pikachu manga. While it also makes an appearance in the anime, Ash did not capture it.
In the manga
Ash's Fearow was one of the major alterations from the anime in Electric Tale of Pikachu. It was the first Pokémon Ash caught in this canon (except Pikachu, who wasn't literally "caught").
Fearow was a leader of a group of Spearow, which attacked and exhausted Ash's Pikachu. However, Pikachu beat Fearow with a Thunder, so Ash was able to catch it, turning Fearow into one of his most used Pokémon in manga, staying with him all the way. In a rather humorous moment, Ash attempted to have it battle against Misty at the Cerulean Gym (during which he had to get his hat back from her), but it came out with a sign around its neck saying "Sleeping. Do not disturb." Only when Ash throws a soy sauce-covered dumpling (its favorite food) at Misty does it actually do anything.
In the anime
In the anime, the leader of the Spearow flock that attacked Ash and Pikachu evolved into Fearow at some time during Ash's journey through Kanto. When he returned to Pallet Town in Pallet Party Panic, this Fearow was again causing trouble, this time for a flock of Pidgey and Pidgeotto. After his own Pidgeotto became a Pidgeot, Ash attempted to capture the Fearow while riding on Pidgeot's back. Despite its weakened state, he was still unable to capture it, and left Pidgeot behind to guard against any future attacks.
For more information on this Pokémon's species, see Spearow and Fearow.