A cent is a penny. 5 cents is a nickel. 10 cents is a dime. If the US mint made a 15 cent piece, what would you name it?
Quent. Qu for the prefix of the Spanish word for 15 (quince`?) Ent for the suffix of cent. I dunno, sounds good @ 2:00am.
Some call it a penny, but it's correct word is a cent, penny is for the UK. Penny is old english. The nickel is named so because it's made out of nickel. The dime comes from disme, french for "tenth". The quarter is a fraction, 1/4 of a dollar. If the 15 cent coin was made out of nickel, you couldn't call it a nickel. Maybe if it was magnesium plated, like the dollar coin, you could call it a "maggy" Since it's 15, you could call it a "teeny".
In reality, I don't think the US would ever make a 15 cent coin. Besides the 3 cent coin, all coins have been made to change for a dollar.
You know, we already had a 15-cent "item," in fact, several of them -- 15c fractional currency, Fourth Issue, with the "Bust of Columbia," as Friedberg #1267-1271 describes them.