At some point in his past he had joined the Beast Pirates under the command of Kaido, and was given an artificial Devil Fruit called a SMILE that gave him the power to transform his hands into the horns of the sheep.
Attack on Zou
Although Sheepshead had the respect and loyalty of his subordinates, he himself was under the command of Jack, one of Kaido's All-Stars, who led his forces to Mokomo Dukedom in order to find and capture Raizo, a rebel samurai from Wano who was a member of the Nine Red Scabbards loyal to the Kozuki clan. Sheepshead mocked the minks living and demanded they show respect for Jack, but was quickly surprised to see other minks easily fighting back, and Sheepshead joined the battle alongside the Gifters.
For five days, Jack's forces, including Sheepshead, fought against the two mink factions and their leaders (Inuarashi and Nekomamushi) and almost lost against the minks, until Jack brought out Koro, the gas weapon created by Caesar Clown, and destroyed the city and several of the minks present.
One day later, Jack learned of the defeat and capture of Donquixote Doflamingo, the black market dealer who manufactured the SMILE Devil Fruits with Caesar Clown for the Beast Pirates, and left Zou to attempt a rescue, leaving Sheepshead in charge of the search for Raizo. During that search, Sheepshead chased a mink, Tristan, but the latter was rescued by Sanji, Nami, Brook, and Chopper of the Straw Hat Pirates (who were also accompanied by Momonosuke and a captive Caesar Clown). Sheepshead later attacked them demanding to know what they did with Tristan, Sanji and Brook managed to get the upper hand and defeat him, leaving Ginrummy in charge of the futile search for Raizo.
Once they believed Raizo was not, in fact, on Zou, Sheepshead and his group reported to an already-angry Jack, and the latter became even angrier with this news, beating up Sheepshead and hanging him upside down by his side. Once their ship approached the giant elephant carrying the Mokomo Dukedom on it's back, Jack's forces attacked but were quickly struck down by Zunesha's trunk, destroying the ship and presumably killing all but Jack.
Sheepshead was sadistic and violent, both traits he proudly displayed in the heat of battle. He also believed himself superior to others, excluding Kaido and the All-Stars, and mocked the minks for not giving proper respect to his superiors.