Cetus in Perseus and Andromeda
Actually, we take that back. Release Cetus!
Cetus is the giant sea monster who is coming to chow down on Andromeda. Unfortunately (for Cetus at least), Perseus slays him before he gets to have his tasty dinner. The sea monster Heracles killed was also called Cetus. Today Cetus is the name of a constellation, which is usually represented by a whale.
For more on Cetus, check out our discussion of him as a symbol.