A female bull is usually called a cow. Often, people think of bulls and cows only in terms of cattle. However, there are other animals whose males and females are termed bulls and cows, respectively.
A male alligator is called a bull, and a female alligator is a cow. Similarly, a male elephant is a bull, and a female elephant is a cow. The same is true of camels, elk, giraffes, gnus, hippopotami, porpoises and dolphins, rhinoceri, seals, walruses, whales, yaks and even dinosaurs.
Male chinchillas are bulls, but females are sows or velvets. A male shark is also a bull, but the female is just referred to as a female shark.