Shark

The Great White Shark is a powerful Treasure Guardian in Age of Empires III: The Asian Dynasties, capable of inflicting heavy damage on early exploration vessels or destroying them if the player gets careless. It is currently the only shark actually named in any of the games, however there are other species which are quite capable of attacking small ships and people which may appear in upcoming titles such as Age of Empires Online.

A single shark may sometimes guard a treasure, more commonly however the treasure will be guarded by a pair of the fish.

Age of Mythology
The Nereid unit from Age of Mythology:The Titans rides a shark as well, it is a cheap and effective myth unit that can stand it's own against most units.

It is unclear what species of shark it actually rides. A large Bull Shark could accomplish the same task of carrying a rider.