This Forretress is a bug/steel-type Pokémon that Brock caught as a Pineco.
As a Pineco, it often would explode around Brock, but after evolving into Forretress in Entei At Your Own Risk, it stopped doing so.
Forretress was the only one of Brock's Pokémon that he took to Hoenn with him and resided as a vital member of his team until Sinnoh. It is currently at the Pewter City Gym.
Using Self-Destruct as Pineco