The pig piggy bank is so special in some countries that there are stamps with their image.
Did you ever notice that publications in the media (TV, newspaper, magazines) on money, economy, cuts in wages/ old age pension (or otherwise) are always illustrated with a pig piggy bank? This is because in the minds of people saving is connected with the image of a pig piggy bank. So a pig, not just another bank! This is (for me) an illustration of the unicity of the pig piggy bank.