Fish (mob)

Fish is a planned passive mob. In the Halloween update, Notch said he would add fish. However, he had limited time, and was thus only able to add them as an item. While there may be coding for the fish in the game itself, there is not a model that currently exists. Most players, however, are convinced that fish will never come as a mob, since it has been two years that the mob has been "planned."

Notch states in the reference the 'ability to catch fish' which is already implemented. Nothing says anything about adding a model.

If you take minecraft.jar apart, you will find entityfish.class.