What is the name of Pokémon's famous yellow mouse mascot?

Pokémon's famous yellow mouse mascot is Pikachu.

Pikachu is an Electric-type Pokémon created by Atsuko Nishida, with the final design completed by Ken Sugimori. Its yellow body, red cheek sacs, and lightning-bolt-shaped tail made it visually distinctive in games and merchandise.

Pikachu became especially famous through the anime, where it accompanied Ash Ketchum. Unlike most Pokémon, Ash's Pikachu usually traveled outside its Poké Ball, helping establish the character as the public face of the franchise.

Pikachu was not one of the three selectable starters in the original Red and Green games. Its mascot role expanded through the anime, Pokémon Yellow, trading cards, toys, films, and later games. Pichu is its baby pre-evolution, while Raichu is its evolved form.