Pikachu is the only starter Pokémon given to Ash in the first anime episode.
In “Pokémon, I Choose You!”, Ash arrives late at Professor Oak’s laboratory and finds that the standard starter Pokémon are already gone. Oak therefore gives him a Pikachu, which initially resists entering its Poké Ball and refuses to obey Ash.
Pikachu is an Electric-type Pokémon and became the central mascot of the animated series. Although Bulbasaur, Charmander, and Squirtle are the three original game starters, Pikachu is not one of those three choices in Pokémon Red and Green.
The episode uses Pikachu’s resistance to create the first major bond between Ash and his partner. Their relationship develops when Ash puts Pikachu’s safety before his own, helping establish the emotional pattern that shaped the long-running television series.