When does forretress learn stealth rock in Pokemon black and white?

Answer 1

In Pokémon Black and Pokémon White, Forretress is not capable of learning Stealth Rock through level up or through the use of TMs, HMs or Tutors. Stealth Rock is an Egg Move for Forretress so you would have to use a female Pineco or Forretress and place her in the Daycare and then you would have to use a male Pokémon such as Dwebble or Crustle that would know Stealth Rock and place him in the Daycare and the Daycare Man will give you an Egg that will result into it hatching into Pineco that knows Stealth Rock.