penny bank vs piggy bank

penny bank and piggy bank both are nouns.

Word	Noun	Adjective	Verb	Adverb
penny bank	Yes	No	No	No
piggy bank	Yes	No	No	No

As nouns, penny bank and piggy bank are synonyms defined as:

penny bank and piggy bank: a child's coin bank (often shaped like a pig)

penny bank (noun)	piggy bank (noun)
a child's coin bank (often shaped like a pig)	a child's coin bank (often shaped like a pig)