Kaidou

Kaidou is the captain of the Beasts Pirates and one of the Yonkou, who was the third to be mentioned by name. Kaidou is known as the "strongest creature in the world." Kaidou's name is first mentioned by Gekko Moriah shortly after having obtained Luffy's shadow. His title is revealed just after the Straw Hats defeat Moriah.

It is said that he has an army of 500 artificial Zoan Devil Fruit users.

Bounty:
4,611,100,000

He stopped being a Yonko after he and Big Mom were beaten by the Worst Generation.
(Source: Wikia)