Pikachu is an Electric-type Pokémon introduced in Generation I. It is also known as the 'Mouse Pokemon'.
How to Obtain
Pikachu could be obtained in the grass patch at the top of Route 1 where a NPC was standing in the grass (old map), or Viridian Forest. In the current map it can be encountered in the patch of grass an NPC is facing (note that the NPC doesn't battle as it looks for Pikachu). It can also be obtained by evolving Pichu, with a Friendship Ribbon, trading, or from the Pokémon Roulette.