Whackas are creatures known for their valuable Whacka Bumps. However, to obtain one from it, one must hit it with a hammer, hurting it. The bumps aren't infinite. After some bashes, the Whacka disappears.
Whacka appears as a Catch Card in Super Paper Mario that you can earn from the card shop or by getting 300 shop points.
Whacka is found on Mt Rugged, and his bumps will restore lots of statistics, so save them for boss battles.