Muscular Pikachu

Pikachu (Japanese: ピカチュウ Pikachu)
is an Electric-type Pokémon.It evolves from Pichu when leveled up with high friendship and evolves into Raichu when exposed to a Thunder Stone. However, the starter Pikachu in Pokémon Yellow will refuse to evolve into Raichu unless it is traded and evolved on another save file.Pikachu is popularly known as the mascot of the Pokémon franchise and a major representative of Nintendo's collective mascots.

In the series, Ash was given the opportunity to use a Thunder Stone to make Pikachu evolve into Raichu in the episode Electric Shock Showdown. Pikachu chose not to evolve in order to demonstrate that you don't have to become something else in order to overcome obstacles.
but when u dont evolve your pikachu it will gets BIG!!!!