There is no animal called "a piece of cake" in the zodiac, so this proverb does not represent any zodiac. It is just a common Chinese expression to describe something very easy or simple.

When people face a task or challenge, if they think it is very easy to solve, they will say "piece of cake" to show that the problem is very simple and easy to solve. Therefore, "a piece of cake" does not represent any Chinese zodiac, but an expression.