In the Pokémon TV series, Zubat is the first Pokémon Brock catches after leaving Pewter City.
Brock begins the anime as the Pewter City Gym Leader and travels with Ash after the pair meet in Kanto. During the journey, Brock catches Zubat in Mt. Moon. The capture expands his team beyond the Rock-type Pokémon associated with his Gym and gives him a Pokémon suited to caves and dark environments.
Zubat later evolves into Golbat and eventually Crobat after Brock travels through later regions. Brock’s Zubat is also useful for locating people and Pokémon in places where visibility is poor. Its presence helps distinguish Brock’s travelling team from the Pokémon he uses at the Pewter Gym.
A common mistake is Onix, Brock’s most recognizable Pokémon in early episodes. Onix is already part of Brock’s Gym team before the journey begins, whereas Zubat is the first Pokémon Brock catches during his travels with Ash.