The following is an excerpt from "The Book of Totally Useless Information":

Why are piggy banks shaped like pigs?

Pigs have recently lost the stigma of being dirty, lazy creatures. Today, there is a pig mania sweeping the country. People are buying everything from calendars with a pig motif to miniature pigs as house pets. The creation of the piggy bank was, however, purely accidental, as many great creations are.

In ancient times a lump of clay was called "pygg." Clay bowls were fashioned from this pygg. People would keep their coins in these bowls and they became known as pygg bowl banks. As the story goes, later in history an order was placed with a potter form several of these pygg bowl banks. The potter, unfamiliar with the term, made ceramic banks shaped like pigs. They were a hit and became known as piggy banks. It is only appropriate that some of the bacon we bring home is dropped into a bank shaped like a pig.