Dalton used to be a guard who served under King Wapol, but was opposed to Wapol's treatment of his subjects. He was seen with Wapol in Alabasta in a meeting with Princess Nefertari Vivi's father. He was inspired to rebel by Hiruluk. After seeing Hirluk's noble sacrifice, he turned against Wapol, but was subdued and thrown in prison, constantly mocked by Wapol and his henchmen, Chess and Kuromarimo. After Wapol abandoned his kingdom when Blackbeard invaded, Dalton took charge and kept the island from falling into disarray, soon gaining the trust of the people.

When Monkey D. Luffy and his crew came to Drum Island in search of a doctor to cure Nami of her illness during the mission to get Princess Nefertari Vivi to Alabasta Kingdom, they encountered Dalton and some men who thought they were here to rob the island. When asked about a doctor, the only one who could heal Nami was a witch named Dr. Kureha.

Even after Wapol returned and once again subdued him, the people and even the top 20 doctors continued to help out Dalton. He was later seriously wounded by Wapol because he defended the townsfolk from arrows lauched by one of Wapol's bodyguards. And he in turn then, even almost going to extreme measures such as planning to blow up Wapol's castle. Luckily, Luffy defeated Wapol and knocked him off the island, after which the people of Drum elected Dalton as their new king and renamed the island, Sakura Kingdom (inspiried from the special chemical created by Hilruk, which turned the snow pink).

He appears later in chapter 440, where he shows Doctor Kureha the new bounties of the Straw Hat Pirates. He doesn't live in the castle, preferring to live in the village.

He has eaten the Devil Fruit known as the Ushi Ushi no Mi, Model: Bison (ウシウシの実 モデル野牛(バイソン) Cow Cow Fruit, Model Bison) which gives him the ability to transform into a bison and into a half human/half bison form that resembles a minotaur. In the English anime the fruit is called the "Ox Ox Fruit" and the animal he turns to is an Ox.

Source: Wikipedia