During the Middle Ages, in about the 15th century, metal was very expensive and seldom used for household wares. Instead, dishes and pots were made of cheap orange clay called pygg. Whenever housewives could save an extra coin, they dropped it into one of their clay jars or pots. They called this their pygg bank.
Over the next two hundred years, people forgot that “pygg” referred to the earthenware material, but the “piggy bank” retained its name and of course people began making clay piggy banks that actually looked like a pig.