The tree-like Ent leader who marches on Isengard in Tolkien’s legendarium is Treebeard.
Treebeard, whose Sindarin name is Fangorn, is one of the oldest beings in Middle-earth. Merry and Pippin meet him in Fangorn Forest after escaping from the Orcs, and he introduces them to the Ents, shepherds of the trees. His deliberation helps bring the Ents into the War of the Ring.
After learning how Saruman’s forces are destroying the forests around Isengard, Treebeard leads an Ent-moot and the Ents attack Saruman’s stronghold. “Treebeard” is his English name, while “Fangorn” can refer both to the character and to the forest. Quickbeam is another Ent, and Old Man Willow is a separate, hostile tree-being in the Old Forest.