Zoo Tycoon's second expansion pack, Marine Mania, includes a secret aquatic animal, the mythical mermaid, that is unlocked within a very unique method that no other animal is.

One of the new scenery pieces included in Marina Mania is a mermaid statue. This statue is place-able in marine tanks, and doing so will the cause the statue to break, revealing a living Mermaid that was trapped inside, which will then function as any other aquatic animal.