The leader of the Uruk-hai that captured Merry and Pippin was Uglúk. Saruman sent Uglúk and his Uruk-hai from Isengard to intercept the Fellowship and bring the two hobbits back to him.
After Boromir’s death, the Uruk-hai carried Merry and Pippin eastward toward Isengard. Uglúk commanded the group and enforced Saruman’s orders, even when the other Orcs wanted to kill or eat the prisoners. His harsh discipline helped keep the captives alive during the pursuit.
The company was eventually attacked by Éomer and the Riders of Rohan near the edge of Fangorn Forest. Uglúk fought fiercely but was killed by Éomer, who beheaded him. Merry and Pippin escaped during the chaos and entered Fangorn, where they met Treebeard.
Gorbag, Shagrat, and Grishnákh are easy mix-ups. Gorbag and Shagrat were Mordor Orc captains associated with the Tower of Cirith Ungol, while Grishnákh was a different Orc leader accompanying the Uruk-hai. Grishnákh pursued Merry and Pippin but did not command Uglúk’s unit.